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/setup.sql.go

@@ -0,0 +1,45 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.29.0
+// source: setup.sql
+
+package sqlite
+
+import (
+	"context"
+)
+
+const initializeSchema = `-- name: InitializeSchema :exec
+CREATE TABLE IF NOT EXISTS content (
+    id TEXT NOT NULL,
+    site_id TEXT NOT NULL,
+    value TEXT NOT NULL,
+    type TEXT NOT NULL CHECK (type IN ('text', 'markdown', 'link')),
+    created_at INTEGER DEFAULT (strftime('%s', 'now')) NOT NULL,
+    updated_at INTEGER DEFAULT (strftime('%s', 'now')) NOT NULL,
+    last_edited_by TEXT DEFAULT 'system' NOT NULL,
+    PRIMARY KEY (id, site_id)
+)
+`
+
+func (q *Queries) InitializeSchema(ctx context.Context) error {
+	_, err := q.db.ExecContext(ctx, initializeSchema)
+	return err
+}
+
+const initializeVersionsTable = `-- name: InitializeVersionsTable :exec
+CREATE TABLE IF NOT EXISTS content_versions (
+    version_id INTEGER PRIMARY KEY AUTOINCREMENT,
+    content_id TEXT NOT NULL,
+    site_id TEXT NOT NULL,
+    value TEXT NOT NULL,
+    type TEXT NOT NULL,
+    created_at INTEGER DEFAULT (strftime('%s', 'now')) NOT NULL,
+    created_by TEXT DEFAULT 'system' NOT NULL
+)
+`
+
+func (q *Queries) InitializeVersionsTable(ctx context.Context) error {
+	_, err := q.db.ExecContext(ctx, initializeVersionsTable)
+	return err
+}