MANUAL RUN: turn on search across repos (CODESPLITTERS_VECTORS=1) and backfill #704

The vector index across repos (#636) ships off. Turning it on takes the owner's deploy:

1. Add CODESPLITTERS_VECTORS=1 to the release script, beside CODESPLITTERS_DEPLOYS=1 CODESPLITTERS_AGENTS=1.
2. plan should show new: Vectorize index codesplitters-vectors (384 dims, cosine), its two metadata indexes (repo, pub), and the Worker's VECTORS and AI bindings. Migration 0052 adds repo_search.stale.
3. Deploy. If wrangler's login lacks ai:write, log in again (rustybuns asks for it now).
4. Backfill: POST /api/admin/scale/search-embed as an admin until left is 0, or let the hourly cron do 200 files an hour.
5. Check: a search across repos with no repo= finds a repo past the 50 most recently committed; the scale sideboard's search card reads "Yes".

Cost: the backfill is the spend, about one embedding per 40 lines of every committed file. Workers AI's free 10k neurons a day cover ~5M tokens; Vectorize on Workers Paid includes 10M stored dimensions (~26k chunks).