Fix FFM-1689: enable cold-start session gate by default

[ohld]

Summary

• Enables COLD_START_NSESSIONS_GATE_ENABLED by default so low-sent dormant returners (nsessions >= 2) fall through to the growing-user blender instead of cold_start_adapt / cold_start_explore.
• Keeps the explicit off-switch behavior covered as an emergency override.
• Adds a default-config regression test proving dormant returners no longer reach cold-start engines.

Root cause

FFM-1161 implemented the nsessions <= 1 cold-start gate behind COLD_START_NSESSIONS_GATE_ENABLED, but the config default stayed False. If production does not explicitly set the env var, the gate is present but inert, matching the observed 7-day dormant-returner leak in FFM-1689.

Guardrail decision

I did not add a new positions 6-10 cold_start_adapt quality guardrail in this PR. The current report says adapt weakness is spread across 21 sources and direct/meme-link cohorts, while existing code already applies a text-light content guard. A source/content quality rule needs a follow-up experiment or Analyst isolation; this PR fixes the obvious routing/config bug first.

Verification

• DATABASE_URL=postgresql+asyncpg://app:app@localhost:65432/app REDIS_URL=redis://localhost:36379/0 CORS_ORIGINS='["http://localhost"]' 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_meme_queue.py tests/feed_turn/test_planner.py -q → 57 passed
• ruff check --fix src/ tests/
• ruff format src/ tests/

Fixes FFM-1689.

[ohld]

STAFF ENGINEER REVIEW: APPROVED — Manual structural review clean. Codex review clean. Targeted verification passed: tests/recommendations/test_meme_queue.py + tests/feed_turn/test_planner.py, 57 passed. CI lint/test are green; proceeding with squash auto-merge.

[ohld]

✅ Approved + merged.