feat(store): video_connections table + RLS + connection store methods

Add the video_connections table (data-model VIDEO_CONNECTION) with FORCE
row-level security keyed off tapir.current_user_id, identical to migration
003's per-user isolation pattern — connections are user-owned data and must
be isolated at the DB layer, not only by application WHERE clauses.

Store methods (UpsertConnection / ConnectionsForUser / DeleteConnection) all
route through withUser so RLS scopes every access. UpsertConnection is
idempotent on (user_id, provider). The OAuth refresh token never lives here;
token_ref is the opaque SecretStore reference.

Extend the RLS isolation proof to cover video_connections: seeded per user,
included in the deny-all + scoped-read assertions, and added to the
cross-user write-invisibility and survivor checks.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/adapters/store/migrations/005_video_connections.down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS video_connections;