fix(topics): URL-safe topic slugs so /topics/<slug> serves (#2099 follow-up)

[jung-thomas]

Problem

All 152 published topic-<slug> BLOBs 404 on DEV /topics/<slug>, even though publish wrote them (verified via /content/hashes). Root cause: flattenTopicSlug only did .replace(/--/g,'-').toLowerCase(), leaving the spaces, colons and slashes that real Tags.titlePath values carry (e.g. software product : technology platform / … / open connectors). /content/topics/:slug delegates to serveHandler, whose VALID_SLUG = /^[a-z0-9][a-z0-9-]*$/ rejects those chars → every topic slug 404s. Concepts work because their slugs are already hyphen-only.

Unit tests passed pre-merge because they used synthetic >/-- titlePaths (clean); prod titlePaths are dirty.

Empirically confirmed on DEV SRV: 0 of 152 stored topic slugs pass VALID_SLUG.

Fix

• Add slugifyTopic(): fold every non-[a-z0-9] run to a single hyphen, trim. flattenTopicSlug delegates to it (subsumes the old -- collapse).
• buildTopicSlugMap slugifies the facet for collision qualification and never emits a leading-hyphen slug when a facetless titlePath collides.
• New unit tests assert real-world dirty titlePaths yield VALID_SLUG-safe slugs; existing collision/normalize tests still pass.

Tree hrefs, stored topic-* keys, and resolveTopicBySlug all derive from buildTopicSlugMap, so the fix propagates uniformly. Tree segment labels stay human-readable (display only).

Deploy note

Requires a full content republish after the fixed backend deploys — render-topics and the tree render server-side, so stored topic-* keys only become URL-safe once this code is live. Column width already proven (current dirty keys are similar length).

Test plan

• test/unit/topic-slug.test.js + topics-query.test.js (18 topic tests green)
• Post-deploy: republish DEV (full) → /topics/<leaf>/ returns 200; topics-route smoke flips 404→200