refactor: implement database-specific schema architecture with schema-as-query pattern

🏗️ **Major Database Schema Refactoring**

**Problem Solved**: Eliminated model duplication and multiple sources of truth by:
- Removed duplicate models (`internal/models/content.go`)
- Replaced inlined schema strings with sqlc-generated setup functions
- Implemented database-specific schemas with proper NOT NULL constraints

**Key Improvements**:
✅ **Single Source of Truth**: Database schemas define all types, no manual sync needed
✅ **Clean Generated Types**: sqlc generates `string` and `int64` instead of `sql.NullString/sql.NullTime`
✅ **Schema-as-Query Pattern**: Setup functions generated by sqlc for type safety
✅ **Database-Specific Optimization**: SQLite INTEGER timestamps, PostgreSQL BIGINT timestamps
✅ **Cross-Database Compatibility**: Single codebase supports both SQLite and PostgreSQL

**Architecture Changes**:
- `db/sqlite/` - SQLite-specific schema and setup queries
- `db/postgresql/` - PostgreSQL-specific schema and setup queries
- `db/queries/` - Cross-database CRUD queries using `sqlc.arg()` syntax
- `internal/db/database.go` - Database abstraction with runtime selection
- `internal/api/models.go` - Clean API models for requests/responses

**Version Control System**: Complete element-level history with user attribution and rollback

**Verification**: ✅ Full API workflow tested (create → update → rollback → versions)
**Production Ready**: Supports SQLite (development) → PostgreSQL (production) migration

justfile

@@ -119,6 +119,10 @@ servedev:
 
 # === Content API Server Commands ===
 
+# Generate Go code from SQL (using sqlc)
+server-generate:
+    cd insertr-server && sqlc generate
+
 # Build the content API server binary
 server-build:
     cd insertr-server && go build -o insertr-server ./cmd/server
@@ -136,6 +140,12 @@ server-health port="8080":
     @echo "🔍 Checking API server health..."
     @curl -s http://localhost:{{port}}/health | jq . || echo "❌ Server not responding at localhost:{{port}}"
 
+# Clean database (development only - removes all content!)
+server-clean-db:
+    @echo "🗑️  Removing development database..."
+    rm -f insertr-server/insertr.db
+    @echo "✅ Database cleaned (will be recreated on next server start)"
+
 # Clean all build artifacts
 clean:
     rm -rf lib/dist