F12: Demo seed data — Seed:Demo flag for dev/demo environments #58

Part of Phase 1.1 (epic #14). A dev-DB wipe currently leaves every screen empty until data is re-clicked by hand; demos start from nothing.

Trigger

• Config flag Seed:Demo (env Seed__Demo=true in deploy/.env, or user-secrets for local debug). Default false — never on unless explicitly set; prod cannot accidentally get fake data.
• DatabaseSeeder runs it after the normal seed, gated on the account having no flocks (same empty-catalog guard as grade seeding): runs once on a fresh DB, no-op on a populated one, never resurrects deleted data.

Seeds

• 2–3 flocks at different ages (e.g. 20wk/45wk layers + one depleted historical flock).
• A handful of customers.
• ~1 week of submitted daily entries per active flock — production by grade, losses, some mortality — so lots, stock, and bird-movement rows all exist through the real domain path (use the handlers/domain methods, not raw inserts).
• One confirmed sales order (FIFO allocation exercised) + one open draft.

Acceptance

• Fresh DB + Seed__Demo=true → every screen (dashboard, daily entry, flocks incl. ledger, stock, customers, sales, history, grades) shows plausible data on first login.
• Second startup: no duplicates (guard respected).
• Flag absent/false → behavior identical to today.