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

TODO.md

@@ -195,3 +195,47 @@ This will immediately unlock:
 - ✅ Full integration between browser editor and CLI enhancement
 
 *Let's build the missing server!*
+
+## 🏗️ **Database Schema Architecture Decision** (Sept 2025)
+
+**Issue**: Inlined SQLite schema in `database.go` creates multiple sources of truth, same problem we just solved with model duplication.
+
+### **Recommended Solutions** (in order of preference):
+
+#### **🎯 Option 1: Schema-as-Query Pattern** ⭐ **RECOMMENDED**
+```
+db/queries/
+├── content.sql          # CRUD queries
+├── versions.sql         # Version control queries  
+├── schema_setup.sql     # Schema initialization as named query
+└── indexes_setup.sql    # Index creation as named query
+```
+
+**Benefits**:
+- ✅ Single source of truth (schema files)
+- ✅ sqlc generates type-safe setup functions  
+- ✅ Consistent with existing sqlc workflow
+- ✅ Database-agnostic parameter syntax (`sqlc.arg()`)
+
+**Implementation**:
+```sql
+-- name: InitializeSchema :exec
+CREATE TABLE IF NOT EXISTS content (...);
+CREATE TABLE IF NOT EXISTS content_versions (...);
+
+-- name: CreateIndexes :exec  
+CREATE INDEX IF NOT EXISTS idx_content_site_id ON content(site_id);
+```
+
+#### **🔧 Option 2: Migration Tool Integration**
+- Use `goose`, `golang-migrate`, or `dbmate`
+- sqlc natively supports parsing migration directories
+- Professional database management with up/down migrations
+- **Trade-off**: Additional dependency and complexity
+
+#### **🗂️ Option 3: Embedded Schema Files**
+- Use `go:embed` to read schema files at compile time
+- Keep schema in separate `.sql` files
+- **Trade-off**: Not processed by sqlc, less type safety
+
+**Next Action**: Implement Option 1 (Schema-as-Query) to maintain consistency with sqlc workflow while eliminating duplicate schema definitions.