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

insertr-server/sqlc.yaml

@@ -0,0 +1,31 @@
+version: "2"
+sql:
+  # SQLite configuration for development
+  - name: "sqlite"
+    engine: "sqlite"
+    queries: ["db/queries/", "db/sqlite/setup.sql"]
+    schema: "db/sqlite/schema.sql"
+    gen:
+      go:
+        package: "sqlite"
+        out: "internal/db/sqlite"
+        emit_json_tags: true
+        emit_prepared_queries: false
+        emit_interface: true
+        emit_exact_table_names: false
+        emit_pointers_for_null_types: false  # All fields are NOT NULL now
+        
+  # PostgreSQL configuration for production
+  - name: "postgresql"
+    engine: "postgresql"
+    queries: ["db/queries/", "db/postgresql/setup.sql"]
+    schema: "db/postgresql/schema.sql"
+    gen:
+      go:
+        package: "postgresql" 
+        out: "internal/db/postgresql"
+        emit_json_tags: true
+        emit_prepared_queries: false
+        emit_interface: true
+        emit_exact_table_names: false
+        emit_pointers_for_null_types: false  # All fields are NOT NULL now