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>

internal/services/db_updater/downloader.go

@@ -7,6 +7,7 @@ import (
 	"os"
 	"path"
 	"thuanle.me/ip-info/configs"
+	"time"
 )
 
 func download(url string) bool {
@@ -16,7 +17,10 @@ func download(url string) bool {
 
 	etag, _ := readFile(filenameEtag)
 
-	client := resty.New()
+	client := resty.New().
+		SetTimeout(2 * time.Minute).
+		SetRetryCount(2).
+		SetRetryWaitTime(5 * time.Second)
 	resp, err := client.R().
 		SetHeader("If-None-Match", etag).
 		Get(url)