Implements the flag/settings backend from docs/discovery/multi-provider/04:
feature_flags + user_settings tables (migration seeds behaviour-preserving
defaults, verified on a fresh real Postgres); IFeatureFlags (15s cache,
FAIL-CLOSED); IAiGate (ai.enabled master flag beats any user opt-in; absent row
= default opt-in). UserSetting tenant-filtered, PK-shared with User. 4 contract
tests; suite 59 green.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>