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/internal/db/sqlite/models.go

@@ -0,0 +1,25 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.29.0
+
+package sqlite
+
+type Content struct {
+	ID           string `json:"id"`
+	SiteID       string `json:"site_id"`
+	Value        string `json:"value"`
+	Type         string `json:"type"`
+	CreatedAt    int64  `json:"created_at"`
+	UpdatedAt    int64  `json:"updated_at"`
+	LastEditedBy string `json:"last_edited_by"`
+}
+
+type ContentVersion struct {
+	VersionID int64  `json:"version_id"`
+	ContentID string `json:"content_id"`
+	SiteID    string `json:"site_id"`
+	Value     string `json:"value"`
+	Type      string `json:"type"`
+	CreatedAt int64  `json:"created_at"`
+	CreatedBy string `json:"created_by"`
+}