feat(publisher-services): implement BE-02 distribution platforms

CHANGELOG.md

@@ -6,6 +6,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 ### Added
+  - [805](https://github.com/thoth-pub/thoth/pull/805) - `BE-02`: implement the approved bounded **distribution platform model** as an inactive additive foundation. Adds the closed 17-value `DistributionPlatform` inventory from `ADR-0004` (no `OTHER`, no fallback, no `Default`, no shared enum or conversion with Thoth Metrics), code-owned compile-time-exhaustive platform descriptors, the PostgreSQL `distribution_platform` enum and the additive `publisher_distribution_platform` relation (composite primary key `(publisher_id, platform)`, `ON DELETE CASCADE` foreign key to `publisher`, named `enabled == (disabled_at IS NULL)` check constraint, partial enabled index and `set_updated_at` trigger), with `thoth-api/src/schema.rs` updated manually and atomically in the same PR under `ADR-0003` Architecture A. Implements the six-transition assignment activation lifecycle (retained disabled rows, application-generated activation UUIDs, one transaction timestamp per transition, same-state no-ops that move no timestamp) and atomic OAPEN/DOAB linked normalization that repairs one-sided, split-activation and split-timestamp pairs rather than treating them as idempotent; `OCLC_KB` and `EX_LIBRIS_KB` stay independently assignable, and `JISC_NBK` is included but inactive and non-assignable, failing closed before any write through the new stable `ThothError::DistributionPlatformNotAssignable` variant. Adds exactly four public additive GraphQL read surfaces — `distributionPlatformOptions`, `publishersByDistributionPlatform`, `publisherCountByDistributionPlatform` and `Publisher.distributionPlatforms` — with two new object types and three new enums, and **no** new mutation, input, scalar or interface; activation IDs, disabled history, adapter/feed identity, package/capability state and protected configuration are not exposed. `Publisher.distributionPlatforms` is the **first production consumer of the `ADR-0007` request-local non-cached DataLoader foundation**: a typed loader added to the existing `RequestLoaders` bundle, keyed on `publisher_id`, built through `configured_loader` with explicit `200`/`10`, loader-first at resolver entry, `try_load` only, total fail-closed batches, and one set-based `eq_any` statement per dispatch chunk executed entirely inside `tokio::task::spawn_blocking`. **Inactive foundation only: no distribution is activated, no distribution job or dissemination exists, the migration creates zero assignment rows, no production migration, deployment, backfill or assignment is performed, mutation-guard mode is unchanged, and no `BE-03`, `BE-04`, `MIG-01`, OAI, Metrics or PR [799](https://github.com/thoth-pub/thoth/pull/799) work is included.**
   - [788](https://github.com/thoth-pub/thoth/pull/788) - `BE-02-SPEC`: reconcile the bounded BE-02 distribution-platform model specification against repository-authoritative `ADR-0007` and the merged request-scoped non-cached DataLoader foundation; replace the obsolete N+1 architecture escalation with the first-production-consumer loader-first contract (request-local non-cached loader, explicit `200`/`10`, `try_load`, total fail-closed batches, set-based Diesel inside `spawn_blocking`, and real-SQL `250 -> [200, 50] -> 2 statements` evidence), while preserving the approved 17-value distribution-platform inventory, assignment lifecycle, linked OAPEN/DOAB normalization, migration, public GraphQL API and rollback decisions. Documentation/control only - no runtime, Cargo, GraphQL implementation, schema implementation, migration implementation, workflow or infrastructure change; no implementation branch; no deployment or production action; no BE-03, BE-04, MIG-01, dissemination, Metrics, mutation-guard or PR #799 action. BE-02 implementation remains **NOT AUTHORIZED** pending fresh independent exact-head specification review, CTO specification approval, merge/repository authority and then separate fresh-base implementation authorization.
   - [802](https://github.com/thoth-pub/thoth/pull/802) - `THOTH-GQL-DATALOADER-01`: implement the `ADR-0007` B0 request-scoped GraphQL **DataLoader foundation** and retire the superseded `ADR-0006` A2 batching machinery. Adds the pinned `dataloader 0.18.0` dependency (default features off, Tokio runtime support on) and a direct `tokio` (`rt`) dependency to `thoth-api`; a request-local, **non-cached** loader bundle (`RequestLoaders`) owned directly by the real GraphQL `Context` and dropped with it, with explicit `200` max-batch-size / `10` yield-count construction, `try_load`-only load API, total fail-closed batch functions, set-based Diesel (`eq_any`) behind a `tokio::task::spawn_blocking` boundary with no connection across `.await`, and a safe cloneable non-serde batch-error projection preserving each field family's current GraphQL error convention. Migrates the general GraphQL unit-test execution path from `juniper::execute_sync` to async Juniper through one bounded central bridge with explicit nested-runtime misuse failure. Removes the unused `ADR-0006` A2 infrastructure (`GraphqlBatchStore`/batching store, look-ahead prefetch, response-scope shim, A2 fixtures and A2-specific tests) and decouples loader availability from mutation-guard mode (`MutationGuardMode::store_available()` removed; `Context` construction no longer takes a guard mode), rehosting the guard/query-path/baseline/directive/duplicate-mutation regression evidence onto A2-independent fixtures. **Adopts no production GraphQL child field; changes no public SDL; performs no database or data migration; changes no guard mode (production remains `OFF`); activates no `OBSERVE`/`ENFORCE`; implements no `BE-02` or Thoth Metrics adoption; deploys nothing.** The pinned-Juniper duplicate top-level mutation execution finding remains a live, separately controlled concern and is **not** fixed by this change
   - [801](https://github.com/thoth-pub/thoth/pull/801) - `THOTH-GQL-DATALOADER-SPEC-01`: propose the `THOTH-GQL-DATALOADER-01` implementation specification for the `ADR-0007` B0 request-scoped GraphQL **DataLoader foundation** and `ADR-0006` A2 retirement (`docs/engineering/ai-delivery/tasks/THOTH-GQL-DATALOADER-01.md`). **Specification and control record only — no runtime, Cargo, GraphQL, schema, migration, workflow or infrastructure change, no implementation, and no implementation branch created.** The proposed specification carries the approved `ADR-0007` architecture into a bounded implementable task: request-local non-cached `dataloader 0.18.x` loaders owned by the real GraphQL `Context`, async Juniper as the supported test/resolver execution model with one bounded central test bridge replacing `execute_sync`, explicit `200`/`10` batching configuration, `try_load`-only load API with total fail-closed batch functions, set-based Diesel behind a `tokio::task::spawn_blocking` boundary with no connection across `.await`, a safe non-panicking shareable batch-error representation (the spike's serde round-trip clone is prohibited as production plumbing), retirement of the unused A2 `GraphqlBatchStore`/prefetch/scope infrastructure, decoupling of batching availability from mutation-guard mode, and rehosting of the guard/duplicate-mutation regression evidence onto A2-independent fixtures. The foundation adopts **no** production GraphQL field and must keep production SDL byte-identical. Status is `APPROVED`: after independent exact-head review (`CHANGES REQUIRED`), bounded remediation of the three review findings (`query_path` preservation split, `mutation_guard.rs` boundary/`store_available()` removal, explicit observability/operations disposition) and fresh review of the corrected content, the CTO granted **specification approval only** on 2026-08-11 at exact PR head `bb4f57e0d446270b4c404879ef61b5e229e90bbf`; `THOTH-GQL-DATALOADER-01` implementation remains **NOT AUTHORIZED** — implementation authorization is a distinct later CTO decision after the approved specification is repository-authoritative on `develop`; `BE-02` and Thoth Metrics adoption, `OBSERVE`/`ENFORCE` activation, and PR [799](https://github.com/thoth-pub/thoth/pull/799)/`OPS-03` all remain outside scope and unauthorized.