feat: scaffold app skeleton with config, ULID, health endpoints, compose stack (phase 1)

- internal .env loader (real env wins) + strictly validated config struct
- ULID package: 48-bit ms timestamp + 80-bit crypto randomness, sortable
- HTTP server with recovery/request-id/trusted-proxy/access-log middleware
- /healthz, /readyz (pluggable checkers), /metricsz (counter registry)
- multi-stage Dockerfile (alpine, non-root) + compose with healthchecks,
  app bound to 127.0.0.1:8787, mongo unpublished, graceful 15s shutdown

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,35 @@
+services:
+  mongo:
+    image: mongo:7
+    restart: unless-stopped
+    # Mongo is reachable only on the compose network; never published to the
+    # host (§12).
+    volumes:
+      - mongo-data:/data/db
+    healthcheck:
+      test: ["CMD", "mongosh", "--quiet", "--eval", "db.adminCommand('ping').ok"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+      start_period: 15s
+
+  app:
+    build: .
+    restart: unless-stopped
+    env_file: .env
+    environment:
+      MONGO_URI: mongodb://mongo:27017
+    ports:
+      - "127.0.0.1:${APP_PORT:-8787}:8787"
+    depends_on:
+      mongo:
+        condition: service_healthy
+    healthcheck:
+      test: ["CMD", "wget", "-qO-", "http://127.0.0.1:8787/healthz"]
+      interval: 10s
+      timeout: 5s
+      retries: 3
+      start_period: 10s
+
+volumes:
+  mongo-data: