feat(db): #71: add versioned SQL migration module and clean up repository DDL

[imdeepmind]

Summary

Replaces ad-hoc per-repository table DDL (initializeTable / InitializeSchema) with a dedicated, versioned SQL migration module under internal/migrations/.

Changes Included

1. Dedicated Migration Package (internal/migrations/):

   • SQLite DDL in internal/migrations/sqlite/001_initial_schema.sql
   • PostgreSQL DDL in internal/migrations/postgresql/001_initial_schema.sql
   • Transactional, idempotent migration runner in internal/migrations/runner.go that maintains the schema_migrations table.

2. Connection Hook:

   • Added Connection.SetAndMigrate(ctx, client, runner) in internal/clients/db/client.go so migrations run whenever a database connection is opened at login or registration.

3. Repository DDL Cleanup:

   • Removed all table creation SQL statements, initMu, and initClient fields from auth, queue, upload, and home repositories.

4. Wiring:

   • Injected migration runner in main.go and auth.Service.