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/db/queries/versions.sql

@@ -0,0 +1,29 @@
+-- name: CreateContentVersion :exec
+INSERT INTO content_versions (content_id, site_id, value, type, created_by)
+VALUES (sqlc.arg(content_id), sqlc.arg(site_id), sqlc.arg(value), sqlc.arg(type), sqlc.arg(created_by));
+
+-- name: GetContentVersionHistory :many
+SELECT version_id, content_id, site_id, value, type, created_at, created_by
+FROM content_versions
+WHERE content_id = sqlc.arg(content_id) AND site_id = sqlc.arg(site_id)
+ORDER BY created_at DESC
+LIMIT sqlc.arg(limit_count);
+
+-- name: GetContentVersion :one
+SELECT version_id, content_id, site_id, value, type, created_at, created_by
+FROM content_versions
+WHERE version_id = sqlc.arg(version_id);
+
+-- name: GetAllVersionsForSite :many
+SELECT 
+    cv.version_id, cv.content_id, cv.site_id, cv.value, cv.type, cv.created_at, cv.created_by,
+    c.value as current_value
+FROM content_versions cv
+LEFT JOIN content c ON cv.content_id = c.id AND cv.site_id = c.site_id
+WHERE cv.site_id = sqlc.arg(site_id)
+ORDER BY cv.created_at DESC
+LIMIT sqlc.arg(limit_count);
+
+-- name: DeleteOldVersions :exec
+DELETE FROM content_versions 
+WHERE created_at < sqlc.arg(created_before) AND site_id = sqlc.arg(site_id);