feat(backend): implement db helpers and postgres pool/transactor

- Add is_blocked to core.users (002_core_tables.sql)
- Add is_active to activity.sessions for soft deletes (005_activity_tables.sql)
- Implement UserRepo: List, GetByID, GetByName, Create, Update, Delete
- Implement MimeRepo: List, GetByID, GetByName
- Implement SessionRepo: Create, GetByTokenHash, ListByUser,
  UpdateLastActivity, Delete, DeleteByUserID
- Session deletes are soft (SET is_active = false); is_active is a
  SQL-only filter, not mapped to the domain type

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/migrations/005_activity_tables.sql

@@ -16,6 +16,7 @@ CREATE TABLE activity.sessions (
     started_at    timestamptz  NOT NULL DEFAULT statement_timestamp(),
     expires_at    timestamptz,
     last_activity timestamptz  NOT NULL DEFAULT statement_timestamp(),
+    is_active     boolean      NOT NULL DEFAULT true,
 
     CONSTRAINT uni__sessions__token_hash UNIQUE (token_hash)
 );