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/api/models.go

@@ -0,0 +1,52 @@
+package api
+
+import "time"
+
+// API request/response models
+type ContentItem struct {
+	ID           string    `json:"id"`
+	SiteID       string    `json:"site_id"`
+	Value        string    `json:"value"`
+	Type         string    `json:"type"`
+	CreatedAt    time.Time `json:"created_at"`
+	UpdatedAt    time.Time `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 time.Time `json:"created_at"`
+	CreatedBy string    `json:"created_by"`
+}
+
+type ContentResponse struct {
+	Content []ContentItem `json:"content"`
+}
+
+type ContentVersionsResponse struct {
+	Versions []ContentVersion `json:"versions"`
+}
+
+// Request models
+type CreateContentRequest struct {
+	ID        string `json:"id"`
+	SiteID    string `json:"site_id,omitempty"`
+	Value     string `json:"value"`
+	Type      string `json:"type"`
+	CreatedBy string `json:"created_by,omitempty"`
+}
+
+type UpdateContentRequest struct {
+	Value     string `json:"value"`
+	Type      string `json:"type,omitempty"`
+	UpdatedBy string `json:"updated_by,omitempty"`
+}
+
+type RollbackContentRequest struct {
+	VersionID    int64  `json:"version_id"`
+	RolledBackBy string `json:"rolled_back_by,omitempty"`
+}