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/setup.sql.go

@@ -0,0 +1,87 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.29.0
+// source: setup.sql
+
+package postgresql
+
+import (
+	"context"
+)
+
+const createContentSiteIndex = `-- name: CreateContentSiteIndex :exec
+CREATE INDEX IF NOT EXISTS idx_content_site_id ON content(site_id)
+`
+
+func (q *Queries) CreateContentSiteIndex(ctx context.Context) error {
+	_, err := q.db.ExecContext(ctx, createContentSiteIndex)
+	return err
+}
+
+const createContentUpdatedAtIndex = `-- name: CreateContentUpdatedAtIndex :exec
+CREATE INDEX IF NOT EXISTS idx_content_updated_at ON content(updated_at)
+`
+
+func (q *Queries) CreateContentUpdatedAtIndex(ctx context.Context) error {
+	_, err := q.db.ExecContext(ctx, createContentUpdatedAtIndex)
+	return err
+}
+
+const createUpdateFunction = `-- name: CreateUpdateFunction :exec
+CREATE OR REPLACE FUNCTION update_content_timestamp()
+RETURNS TRIGGER AS $$
+BEGIN
+    NEW.updated_at = EXTRACT(EPOCH FROM NOW());
+    RETURN NEW;
+END;
+$$ LANGUAGE plpgsql
+`
+
+func (q *Queries) CreateUpdateFunction(ctx context.Context) error {
+	_, err := q.db.ExecContext(ctx, createUpdateFunction)
+	return err
+}
+
+const createVersionsLookupIndex = `-- name: CreateVersionsLookupIndex :exec
+CREATE INDEX IF NOT EXISTS idx_content_versions_lookup ON content_versions(content_id, site_id, created_at DESC)
+`
+
+func (q *Queries) CreateVersionsLookupIndex(ctx context.Context) error {
+	_, err := q.db.ExecContext(ctx, createVersionsLookupIndex)
+	return err
+}
+
+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 BIGINT DEFAULT (EXTRACT(EPOCH FROM NOW())) NOT NULL,
+    updated_at BIGINT DEFAULT (EXTRACT(EPOCH FROM 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 SERIAL PRIMARY KEY,
+    content_id TEXT NOT NULL,
+    site_id TEXT NOT NULL,
+    value TEXT NOT NULL,
+    type TEXT NOT NULL,
+    created_at BIGINT DEFAULT (EXTRACT(EPOCH FROM 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
+}