fix(clients): backfill missing subId on startup and guard create/update

Legacy clients (and any API consumer that POSTs to AddInboundClient
without a subId) ended up with an empty SubID, which breaks the panel's
sub-link generation. Backfill them once at startup and stop the gap at
the write path so new clients can't reintroduce it.

- util/random: add NumLower(n) — 16-char [0-9a-z] generator that matches
  the frontend's RandomUtil.randomLowerAndNum convention.
- database/db.go: new InboundClientSubIdFix seeder, modeled on
  InboundClientTgIdFix. Loops every inbound, parses settings.clients,
  fills empty/missing subId with random.NumLower(16), persists via the
  same transaction-wrapped Update("settings", …) path, then records in
  HistoryOfSeeders so it runs at most once.
- web/service/client.go: defense-in-depth in AddInboundClient and
  UpdateInboundClient — fill subId on the persisted settings map when
  the payload omits it (Update prefers the previous value before
  generating a fresh one).
- database/db_seed_test: cover empty subId, missing-key subId, and
  preserved-existing subId; assert exactly one HistoryOfSeeders row.

util/random/random.go

@@ -51,6 +51,19 @@ func Seq(n int) string {
 	return string(runes)
 }
 
+// NumLower generates a random string of length n containing digits and lowercase letters only.
+func NumLower(n int) string {
+	runes := make([]rune, n)
+	for i := range n {
+		idx, err := rand.Int(rand.Reader, big.NewInt(int64(len(numLowerSeq))))
+		if err != nil {
+			panic("crypto/rand failed: " + err.Error())
+		}
+		runes[i] = numLowerSeq[idx.Int64()]
+	}
+	return string(runes)
+}
+
 // Num generates a random integer between 0 and n-1.
 func Num(n int) int {
 	bn := big.NewInt(int64(n))