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/onboarding_service.py

@@ -203,6 +203,11 @@ class OnboardingService:
 
         Returns response with next step information.
         """
+        # Check vendor exists BEFORE creating onboarding record (FK constraint)
+        vendor = self.db.query(Vendor).filter(Vendor.id == vendor_id).first()
+        if not vendor:
+            raise VendorNotFoundException(vendor_id)
+
         onboarding = self.get_or_create_onboarding(vendor_id)
 
         # Update onboarding status if this is the first step
@@ -210,11 +215,6 @@ class OnboardingService:
             onboarding.status = OnboardingStatus.IN_PROGRESS.value
             onboarding.started_at = datetime.now(UTC)
 
-        # Get vendor and company
-        vendor = self.db.query(Vendor).filter(Vendor.id == vendor_id).first()
-        if not vendor:
-            raise VendorNotFoundException(vendor_id)
-
         company = vendor.company
 
         # Update company name if provided