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

package.json

@@ -1,7 +1,7 @@
 {
   "name": "insertr",
   "version": "0.1.0",
-  "description": "The Tailwind of CMS - Zero-configuration content editing for any static site",
+  "description": "The Tailwind of CMS - Zero-configuration content editing with version control for any static site",
   "main": "lib/dist/insertr.js",
   "type": "module",
   "scripts": {
@@ -21,10 +21,14 @@
     "headless-cms",
     "static-site-generator",
     "content-management",
+    "version-control",
+    "content-versioning",
     "build-time-enhancement",
     "zero-config",
     "go",
-    "javascript"
+    "javascript",
+    "sqlc",
+    "sqlite"
   ],
   "author": "Insertr Team",
   "license": "MIT",