feat: implement M2 — multiple upstreams & config

- Failover strategy: try resolvers in order until one succeeds
- Env var overrides: DOH_LISTEN, DOH_UPSTREAM_STRATEGY, DOH_UPSTREAM_RESOLVERS
- Graceful shutdown on SIGINT/SIGTERM
- Strategy parsed from config string to enum
- 14 tests passing, clippy clean, fmt clean

DEVELOP.md

@@ -86,7 +86,7 @@ See [docs/plan.md](docs/plan.md) for the full project plan and milestone checkli
 | Milestone | Status |
 |-----------|--------|
 | M1 — Core Forwarding | ✅ Done |
-| M2 — Multiple Upstreams & Config | 🔲 Not started |
+| M2 — Multiple Upstreams & Config | ✅ Done |
 | M3 — Caching | 🔲 Not started |
 | M4 — Observability | 🔲 Not started |
 | M5 — Blocking & Filtering | 🔲 Not started |