refactor: replace universal_migration.py with Alembic

Remove the 7,791-line universal_migration.py and 16 incomplete individual
Alembic migrations. Replace with a single initial schema migration using
Base.metadata.create_all(checkfirst=True).

Changes:
- Add programmatic Alembic runner (app/database/migrations.py) with
  auto-stamp logic for existing databases transitioning from
  universal_migration
- Extract ensure_default_web_api_token() to web_api_token_service.py
- Extract sync_postgres_sequences() to database.py with SQL injection
  prevention via _quote_ident()
- Add HMAC token hashing support with backward-compatible dual-hash
  fallback and automatic rehashing
- Remove dead init_db() function and unused imports
- Add Makefile targets: migrate, migration, migrate-stamp, migrate-history
- Fix fileConfig() destroying structlog config (disable_existing_loggers)
- Remove duplicate migrations/alembic/alembic.ini with credentials
- Add script.py.mako template for future migration generation
- Update startup flow: alembic upgrade → sync sequences → ensure token
- Harden database.py: ParamSpec for retry decorator, safe URL logging,
  echo='debug' mode, execute_with_retry validation
- Update documentation references

31 files changed, 302 insertions(+), 9,226 deletions(-)

alembic.ini

@@ -2,7 +2,7 @@
 script_location = migrations/alembic
 prepend_sys_path = .
 version_path_separator = os
-sqlalchemy.url = postgresql+asyncpg://vpn_user:your_password@localhost:5432/vpn_bot
+sqlalchemy.url = driver://user:pass@localhost/dbname
 
 [post_write_hooks]