refactor: consolidate database structure and move injector to engine

Database Structure Cleanup:
- Move all SQL files from ./db/ to ./internal/db/
- Update sqlc.yaml to use new paths (preserving schema+setup.sql hack)
- Consolidate database-related code in single directory
- Remove empty ./db/ directory

Injector Migration:
- Move injector.go from content package to engine package
- Update ContentClient interface to return map instead of slice for GetBulkContent
- Update database client implementation to match interface
- Remove injector dependency from enhancer (stub implementation)

Demo-Site Consolidation:
- Move demo-site to test-sites/demo-site for better organization
- Update build scripts to use new demo-site location
- Maintain all functionality while improving project structure

This continues the unified architecture consolidation by moving core content
processing logic to the engine and organizing related files properly.

internal/db/postgresql/schema.sql

@@ -0,0 +1,42 @@
+-- PostgreSQL-specific schema with BIGINT UNIX timestamps
+-- Main content table (current versions only)
+CREATE TABLE 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,
+    updated_at BIGINT DEFAULT EXTRACT(EPOCH FROM NOW()) NOT NULL,
+    last_edited_by TEXT DEFAULT 'system' NOT NULL,
+    PRIMARY KEY (id, site_id)
+);
+
+-- Version history table for rollback functionality
+CREATE TABLE 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
+);
+
+-- Indexes for performance
+CREATE INDEX IF NOT EXISTS idx_content_site_id ON content(site_id);
+CREATE INDEX IF NOT EXISTS idx_content_updated_at ON content(updated_at);
+CREATE INDEX IF NOT EXISTS idx_content_versions_lookup ON content_versions(content_id, site_id, created_at DESC);
+
+-- Function and trigger to automatically update updated_at timestamp
+CREATE OR REPLACE FUNCTION update_updated_at_column()
+RETURNS TRIGGER AS $$
+BEGIN
+    NEW.updated_at = EXTRACT(EPOCH FROM NOW());
+    RETURN NEW;
+END;
+$$ language 'plpgsql';
+
+CREATE TRIGGER update_content_updated_at
+    BEFORE UPDATE ON content
+    FOR EACH ROW
+    EXECUTE FUNCTION update_updated_at_column();