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/postgresql/db.go

@@ -0,0 +1,31 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.29.0
+
+package postgresql
+
+import (
+	"context"
+	"database/sql"
+)
+
+type DBTX interface {
+	ExecContext(context.Context, string, ...interface{}) (sql.Result, error)
+	PrepareContext(context.Context, string) (*sql.Stmt, error)
+	QueryContext(context.Context, string, ...interface{}) (*sql.Rows, error)
+	QueryRowContext(context.Context, string, ...interface{}) *sql.Row
+}
+
+func New(db DBTX) *Queries {
+	return &Queries{db: db}
+}
+
+type Queries struct {
+	db DBTX
+}
+
+func (q *Queries) WithTx(tx *sql.Tx) *Queries {
+	return &Queries{
+		db: tx,
+	}
+}