feat: 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/002_core_tables.sql

@@ -6,6 +6,7 @@ CREATE TABLE core.users (
     password   text        NOT NULL,  -- bcrypt hash via pgcrypto
     is_admin   boolean     NOT NULL DEFAULT false,
     can_create boolean     NOT NULL DEFAULT false,
+    is_blocked boolean     NOT NULL DEFAULT false,
 
     CONSTRAINT uni__users__name UNIQUE (name)
 );