Add embedded Vue SPA admin panel with go:embed

This is the complete fix for the missing /admin/ route and frontend serving:

Backend:
- internal/web/web.go: new package with go:embed for web/dist/
- internal/api/router.go: add routes for /admin/, /admin/*, /assets/*
- internal/db/db.go: fix SQLite DSN parsing (sqlite:///path -> path)

Build system:
- Makefile: new 'embed-prep' target copies web/dist to internal/web/dist
- make build now runs embed-prep -> frontend/build automatically

Deployment:
- deploy/llamalink.service: remove invalid --host/--port flags,
  add EnvironmentFile=/etc/llamalink/env

Verified:
- /health returns 200
- /admin/ serves Vue SPA HTML
- /assets/* serves CSS and JS files from embedded FS
- sqlite:///./llamalink.db works correctly

deploy/llamalink.service

@@ -6,23 +6,20 @@ After=network.target
 Type=simple
 User=llamalink
 WorkingDirectory=/opt/llamalink
-ExecStart=/usr/local/bin/llamalink \
-    --host 0.0.0.0 \
-    --port 8000
+EnvironmentFile=/etc/llamalink/env
+ExecStart=/usr/local/bin/llamalink
 Restart=on-failure
 RestartSec=5
 StandardOutput=journal
 StandardError=journal
 SyslogIdentifier=llamalink
 
-# Security
 NoNewPrivileges=true
 PrivateTmp=true
 ProtectSystem=strict
 ProtectHome=true
 ReadWritePaths=/opt/llamalink/data
 ReadOnlyPaths=/opt/llamalink/models
-Environment=LLAMALINK_ENV=production
 
 [Install]
 WantedBy=multi-user.target