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

docs/plan.md

@@ -46,11 +46,11 @@ A lightweight DNS-over-HTTPS (DoH) forwarder that accepts DNS queries over HTTPS
 
 ### M2 — Multiple Upstreams & Config
 
-- [ ] TOML config file support
-- [ ] Multiple upstream resolver support
-- [ ] Upstream selection strategy (round-robin / failover)
-- [ ] Environment variable overrides
-- [ ] Graceful shutdown
+- [x] TOML config file support
+- [x] Multiple upstream resolver support
+- [x] Upstream selection strategy (round-robin / failover)
+- [x] Environment variable overrides
+- [x] Graceful shutdown
 
 ### M3 — Caching