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/main.go

@@ -3,12 +3,22 @@ package db_updater
 import (
 	"github.com/robfig/cron/v3"
 	"github.com/rs/zerolog/log"
+	"sync/atomic"
 	"thuanle.me/ip-info/configs"
 	"thuanle.me/ip-info/internal/data"
 	"time"
 )
 
-var DbUpdatedAt time.Time
+// DbUpdatedAt stores the DB last-updated time as unix milliseconds.
+// Accessed concurrently by the cron goroutine (writer) and the /metrics
+// handler (reader), so it must be synchronized — hence atomic.Int64.
+var DbUpdatedAt atomic.Int64
+
+// DbUpdatedAtMillis returns the last DB update time in unix milliseconds,
+// or 0 if the DB has never been updated.
+func DbUpdatedAtMillis() int64 {
+	return DbUpdatedAt.Load()
+}
 
 func StartUpdateDbService() {
 	c := cron.New()
@@ -25,7 +35,12 @@ func fetchDbs() {
 
 	if newFlag {
 		log.Info().Msg("New DB downloaded. Recreating mmdb")
-		_ = mergeMmdb()
+		if err := mergeMmdb(); err != nil {
+			// Keep the current (previous) DB rather than reloading a
+			// potentially empty/corrupt merge output.
+			log.Err(err).Msg("Failed to merge mmdb, keeping current DB")
+			return
+		}
 
 		err := data.Ins().Reload()
 		if err != nil {
@@ -33,6 +48,6 @@ func fetchDbs() {
 			return
 		}
 
-		DbUpdatedAt = time.Now()
+		DbUpdatedAt.Store(time.Now().UnixMilli())
 	}
 }