fix: comprehensive security, performance, and quality hardening

Security: apply AdminOnly middleware to mutating routes, require
ENCRYPTION_KEY and ADMIN_PASSWORD (no insecure defaults), restrict
CORS to same-origin, fix OIDC token delivery via cookie instead of
URL query param, add rate limiting on login, add MaxBytesReader,
validate volume paths against traversal, add security headers,
validate user roles, add Secure flag to OIDC cookie.

Performance: set SQLite MaxOpenConns(1) to prevent SQLITE_BUSY,
add FK indexes on 8 columns, track notifier goroutines with
WaitGroup for graceful shutdown, use GetRegistryByName instead of
GetAllRegistries in deployer, pass basePath param to avoid redundant
settings query, return empty slices from store to remove reflection.

Quality: refactor TriggerDeploy to delegate to runDeploy (~100 lines
removed), consolidate duplicated utilities (extractPort, boolToInt,
now, isTerminalStatus) into shared exports, migrate all log.Printf
to slog structured logging, use consistent webhook response envelope,
remove dead code (parseEnvVars, duplicate auth types).

UX: clean up NPM proxy on instance removal via API, add README with
quickstart guide, add .env.example, require ADMIN_PASSWORD in
docker-compose, document staging-net prerequisite.

docker-compose.yml

@@ -16,8 +16,8 @@ services:
     environment:
       # Required: protects all credentials stored in the database.
       - ENCRYPTION_KEY=${ENCRYPTION_KEY:?Set ENCRYPTION_KEY in .env}
-      # Optional: default admin password on first launch (default: "admin").
-      - ADMIN_PASSWORD=${ADMIN_PASSWORD:-admin}
+      # Required on first launch: password for the default admin user.
+      - ADMIN_PASSWORD=${ADMIN_PASSWORD:?Set ADMIN_PASSWORD in .env}
       # Optional: override seed file location.
       - SEED_FILE=/app/docker-watcher.yaml
       # Optional: override data directory.
@@ -41,6 +41,8 @@ volumes:
   docker-watcher-data:
     driver: local
 
+# NOTE: The staging-net network must exist before starting.
+# Create it with: docker network create staging-net
 networks:
   staging-net:
     external: true