config: unify configuration with multi-site support

- Remove mock_content setting (working database loop makes it unnecessary)
- Change server.dev_mode to global dev_mode setting for consistency
- Update CLI to use cli.site_id and cli.output for scoped configuration
- Implement database client for CLI enhance command (complete static site loop)
- Update justfile to use INSERTR_DATABASE_PATH environment variable
- Enable multi-site architecture: server is site-agnostic, CLI is site-specific
- Unified insertr.yaml now supports both server and CLI with minimal config

justfile

@@ -34,7 +34,7 @@ dev: build-lib build
     
     # Start API server with prefixed output
     echo "🔌 Starting API server (localhost:8080)..."
-    ./insertr serve --dev-mode --db ./dev.db 2>&1 | sed 's/^/🔌 [SERVER] /' &
+    INSERTR_DATABASE_PATH=./dev.db ./insertr serve --dev-mode 2>&1 | sed 's/^/🔌 [SERVER] /' &
     SERVER_PID=$!
     
     # Wait for server startup
@@ -70,7 +70,7 @@ demo-only:
 dev-about: build-lib build
     #!/usr/bin/env bash
     echo "🚀 Starting full-stack development (about page)..."
-    ./insertr serve --dev-mode --db ./dev.db &
+    INSERTR_DATABASE_PATH=./dev.db ./insertr serve --dev-mode &
     SERVER_PID=$!
     sleep 3
     npx --prefer-offline live-server demo-site --port=3000 --host=localhost --open=/about.html
@@ -120,15 +120,15 @@ enhance input="demo-site" output="dist":
 
 # Start content API server (default port 8080)
 serve port="8080":
-    ./insertr serve --port {{port}} --dev-mode --db ./dev.db
+    INSERTR_DATABASE_PATH=./dev.db ./insertr serve --port {{port}} --dev-mode
 
 # Start API server in production mode
 serve-prod port="8080" db="./insertr.db":
-    ./insertr serve --port {{port}} --db {{db}}
+    INSERTR_DATABASE_PATH={{db}} ./insertr serve --port {{port}}
 
 # Start API server with auto-restart on Go file changes
 serve-dev port="8080":
-    find . -name "*.go" | entr -r ./insertr serve --port {{port}} --dev-mode --db ./dev.db
+    find . -name "*.go" | entr -r bash -c 'INSERTR_DATABASE_PATH=./dev.db ./insertr serve --port {{port}} --dev-mode'
 
 # Check API server health
 health port="8080":