Disable inline share canary by default

[ohld]

Summary

• Set TELEGRAM_INLINE_SHARE_CANARY_PERCENT default to 0 so future deployments do not expose new exact-meme inline share assignments unless explicitly re-enabled.
• Kept existing assignment/history logic intact; the runtime guard already returns url_share when the canary percent is zero.

Production action

• Added TELEGRAM_INLINE_SHARE_CANARY_PERCENT=0 to the Coolify ffmemes-backend application env.
• Queued and completed a production redeploy for commit 3c3ce301 so the running app reloads the env setting.

Verification

• ruff check --fix src/ tests/
• ruff format src/ tests/
• DATABASE_URL='postgresql+asyncpg://postgres:postgres@localhost/postgres' REDIS_URL='redis://localhost:6379/0' CORS_ORIGINS='[]' CORS_HEADERS='[]' TELEGRAM_BOT_USERNAME='ffmemesbot' python3 -m pytest tests/tgbot/test_sharing.py -q

Fixes FFM-1745.

[ohld]

STAFF ENGINEER REVIEW: APPROVED — No structural issues found. Scope is limited to changing TELEGRAM_INLINE_SHARE_CANARY_PERCENT default from 20 to 0; existing zero-canary guard returns url_share before experiment reads/assignments. Verified with tests/tgbot/test_sharing.py -q, git diff --check, CI lint/test green, Codex review clean. CSO skipped: no auth/authz/payments/uploads/raw SQL/secrets/integrations/webhook surface touched.

[ohld]

✅ Approved + merged.