F11: Reactivate flock — undo deplete/archive #57

Part of Phase 1.1 (epic #14). Found the hard way (2026-07-16): deplete/archive are one-way — a mis-click is unrecoverable from the UI, and the only "undo" is database surgery. That's a data-loss-inducing UX trap.

Scope

• Flock.Reactivate() — Depleted → Active and Archived → Active; clears DepletedOn/ArchivedOn; 409 when already Active; bumps Version (AGENTS.md rule) + parallel-race test.
• POST /flocks/{id}/reactivate.
• Flocks screen: "reactivate" action on Depleted/Archived rows (where deplete/archive show today).
• Reactivation restores normal capture: daily entries/movements accepted for any date again (the lifecycle stamps are cleared, so the CanRecordProductionOn backfill window disappears with them).

Notes

• Consider a lightweight confirm on deplete/archive in the SPA while here (one mis-click caused a DB wipe) — cheap confirm() is fine at MVP polish level.

Acceptance

• Deplete → reactivate → flock back in daily-entry picker and dashboard, entries recordable for today.
• Archive → reactivate → same.
• Reactivate on Active flock → 409.
• Race test: parallel reactivates, Version advances per success.