Fix data races, robustness, and cleanup across codebase (#10-#21)

Concurrency & correctness:
- #10: guard IpDb Query/IsLoaded with RLock so Reload() (which reassigns d.r
  and closes the old reader) cannot race with concurrent lookups. Confirmed
  via -race regression test.
- #12: replace DbUpdatedAt time.Time with atomic.Int64 (cron writes,
  /metrics reads) to fix the read/write data race.
- #11: graceful HTTP shutdown (srv.Shutdown with 10s timeout) instead of
  srv.Close() aborting in-flight requests.
- #14: stop swallowing mergeMmdb() errors in fetchDbs() — keep the previous
  DB when a merge fails instead of reloading a possibly-empty output.
- #13: add resty timeout (2m) + retry (x2) to download() so a hung CDN can't
  stall the daily cron forever.
- #15: correct HTTP status codes (503 db loading, 404 not found via new
  ErrNotFound sentinel, 500 otherwise) instead of 200 on query error.

Robustness:
- #16: surface osx.Copy dstFile.Close() errors (flush may fail) via named
  return + defer.

Dependency migration:
- #17: migrate maxminddb-golang v1 -> v2. v2 is a breaking API
  (LookupNetwork -> Lookup returning Result, netip.Addr), so Query was
  rewritten; v1 dropped from go.mod.

Tests:
- #21: add internal/data unit tests (valid/invalid/not-found lookup) plus a
  concurrent Query/Reload race regression test. chdir to repo root in TestMain
  because data helpers use relative paths.

Cleanup:
- #18: README Go version 1.22 -> 1.25.
- #19: replace stray fmt.Printf with zerolog in ipdb.go.
- #20: .env.example API_PORT 28080 -> 8080 (container port, matches
  docker-compose 28080:8080 mapping) with an explanatory comment.

Closes #10, #11, #12, #13, #14, #15, #16, #17, #18, #19, #20, #21

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

go.mod

@@ -6,7 +6,7 @@ require (
 	github.com/gin-gonic/gin v1.12.0
 	github.com/go-resty/resty/v2 v2.17.2
 	github.com/joho/godotenv v1.5.1
-	github.com/oschwald/maxminddb-golang v1.13.1
+	github.com/oschwald/maxminddb-golang/v2 v2.4.1
 	github.com/robfig/cron/v3 v3.0.1
 	github.com/rs/zerolog v1.35.1
 	github.com/safing/mmdbmeld v0.3.0
@@ -32,7 +32,6 @@ require (
 	github.com/maxmind/mmdbwriter v1.2.0 // indirect
 	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
 	github.com/modern-go/reflect2 v1.0.2 // indirect
-	github.com/oschwald/maxminddb-golang/v2 v2.4.1 // indirect
 	github.com/pelletier/go-toml/v2 v2.4.3 // indirect
 	github.com/quic-go/qpack v0.6.0 // indirect
 	github.com/quic-go/quic-go v0.60.0 // indirect