chore: PostgreSQL migration compatibility and infrastructure improvements

Database & Migrations:
- Update all Alembic migrations for PostgreSQL compatibility
- Remove SQLite-specific syntax (AUTOINCREMENT, etc.)
- Add database utility helpers for PostgreSQL operations
- Fix services to use PostgreSQL-compatible queries

Documentation:
- Add comprehensive Docker deployment guide
- Add production deployment documentation
- Add infrastructure architecture documentation
- Update database setup guide for PostgreSQL-only
- Expand troubleshooting guide

Architecture & Validation:
- Add migration.yaml rules for SQL compatibility checking
- Enhance validate_architecture.py with migration validation
- Update architecture rules to validate Alembic migrations

Development:
- Fix duplicate install-all target in Makefile
- Add Celery/Redis validation to install.py script
- Add docker-compose.test.yml for CI testing
- Add squash_migrations.py utility script
- Update tests for PostgreSQL compatibility
- Improve test fixtures in conftest.py

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/services/subscription_service.py

@@ -207,6 +207,18 @@ class SubscriptionService:
             )
         return subscription
 
+    def get_current_tier(
+        self, db: Session, vendor_id: int
+    ) -> TierCode | None:
+        """Get vendor's current subscription tier code."""
+        subscription = self.get_subscription(db, vendor_id)
+        if subscription:
+            try:
+                return TierCode(subscription.tier)
+            except ValueError:
+                return None
+        return None
+
     def get_or_create_subscription(
         self,
         db: Session,