Fix: freeze recently liked v2 enrollment

[ohld]

Fixes FFM-1917.

What changed

• Added an explicit freeze gate for recently_liked_blender_v2 new enrollment after the existing assignment read.
• Existing assignment rows still return their saved variant, so Analyst can keep the passive watch on already-assigned users.
• Unassigned mature users now fall back to control mature-feed weights without inserting control/treatment/excluded experiment rows.
• Updated the active experiment record with the freeze timestamp and rollback path.

Root cause

The assignment path still inserted new recently_liked_blender_v2 experiment rows after the July 22 checkpoint showed the 1,000-per-arm gate was no longer reachable in a useful window.

Freeze record

• Freeze recorded at: 2026-07-23T18:05:54Z
• Effective freeze point: after this PR is merged and deployed from production
• Rollback path: set RECENTLY_LIKED_BLENDER_V2_ENROLLMENT_FROZEN = False and redeploy.

Validation

• ruff check --fix src/ tests/
• ruff format src/ tests/
• env DATABASE_URL=postgresql+asyncpg://postgres:postgres@localhost:65432/postgres REDIS_URL=redis://localhost:36379/0 CORS_ORIGINS='["*"]' CORS_HEADERS='["*"]' TELEGRAM_BOT_TOKEN=test TELEGRAM_BOT_USERNAME=test_bot TELEGRAM_BOT_WEBHOOK_SECRET=test MEME_STORAGE_TELEGRAM_CHAT_ID=1 UPLOADED_MEMES_REVIEW_CHAT_ID=2 ADMIN_LOGS_CHAT_ID=3 ENVIRONMENT=testing python3 -m pytest tests/recommendations/test_blender_experiments.py -q

Reviewer notes

Staff Engineer should verify the gate placement preserves existing assignments and does not touch cold-start routing, cold_start_candidate_filter, moderator low-sent quota, or unrelated engines. FFM-1918 remains the Analyst post-deploy passive watch.

[ohld]

STAFF ENGINEER REVIEW: APPROVED — clean structural review. Freeze gate is placed after existing assignment lookup and before metrics/assignment insert, preserving existing variants while preventing new enrollment; new unassigned mature users fall back to control weights without experiment rows. Local targeted tests passed: tests/recommendations/test_blender_experiments.py 16/16. Codex review found no P1 issues. CI lint/test are green.

[ohld]

✅ Approved + merged.