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/content.sql

@@ -0,0 +1,30 @@
+-- name: GetContent :one
+SELECT id, site_id, value, type, created_at, updated_at, last_edited_by
+FROM content 
+WHERE id = sqlc.arg(id) AND site_id = sqlc.arg(site_id);
+
+-- name: GetAllContent :many
+SELECT id, site_id, value, type, created_at, updated_at, last_edited_by
+FROM content 
+WHERE site_id = sqlc.arg(site_id)
+ORDER BY updated_at DESC;
+
+-- name: GetBulkContent :many
+SELECT id, site_id, value, type, created_at, updated_at, last_edited_by
+FROM content 
+WHERE site_id = sqlc.arg(site_id) AND id IN (sqlc.slice('ids'));
+
+-- name: CreateContent :one
+INSERT INTO content (id, site_id, value, type, last_edited_by)
+VALUES (sqlc.arg(id), sqlc.arg(site_id), sqlc.arg(value), sqlc.arg(type), sqlc.arg(last_edited_by))
+RETURNING id, site_id, value, type, created_at, updated_at, last_edited_by;
+
+-- name: UpdateContent :one
+UPDATE content 
+SET value = sqlc.arg(value), type = sqlc.arg(type), last_edited_by = sqlc.arg(last_edited_by)
+WHERE id = sqlc.arg(id) AND site_id = sqlc.arg(site_id)
+RETURNING id, site_id, value, type, created_at, updated_at, last_edited_by;
+
+-- name: DeleteContent :exec
+DELETE FROM content 
+WHERE id = sqlc.arg(id) AND site_id = sqlc.arg(site_id);