feat(api): add GET endpoint to look up clients by Telegram ID (#5945)

* feat(api): add GET endpoint to look up clients by Telegram ID

GET /panel/api/clients/getByTgId/:tgId returns all clients matching the given Telegram user ID. tgId is not unique, so the response is an array of {client, inboundIds, externalLinks, usedTraffic} objects.

* fix: guard tgId=0 sentinel, index tg_id, deduplicate enrichment in getByTgId

Three issues from the code review on the new GET /panel/api/clients/getByTgId/:tgId
endpoint: the lookup did not short-circuit tgId <= 0 (this codebase's sentinel
for 'no Telegram ID'), had no index on clients.tg_id causing a full table scan
on every call, and duplicated the per-record enrichment (inbound IDs, external
links, effective flow, traffic) identically between get and getByTgId.

- Reject tgId <= 0 in GetRecordsByTgId with a clear error, matching the
  '0 = none' convention used elsewhere in the codebase.
- Add index:idx_clients_tg_id to ClientRecord.TgID (struct tag + idempotent
  startup migration for existing databases).
- Extract buildClientPayload helper used by both get and getByTgId.
- Update client_lookup_test.go to verify sentinel rejection instead of
  expecting tgId=0 to be a valid lookup.

* refactor(api): move Telegram client lookup under /get/tgId/:tgId

Nest the Telegram-ID lookup beside the email lookup as /get/tgId/:tgId
instead of the flat /getByTgId/:tgId, so both client fetch routes share the
/get prefix. Gin resolves the static tgId segment ahead of the :email
wildcard, so /get/:email keeps matching plain email lookups, including a
literal 'tgId' email. The endpoint is unreleased, so no compatibility
concern.

frontend/src/pages/api-docs/endpoints.ts

@@ -582,6 +582,16 @@ export const sections: readonly Section[] = [
         response:
           '{\n  "success": true,\n  "obj": {\n    "client": { "id": 1, "email": "alice@example.com", ... },\n    "inboundIds": [3, 5],\n    "externalLinks": [{ "kind": "link", "value": "vless://...", "remark": "DE" }]\n  }\n}',
       },
+      {
+        method: 'GET',
+        path: '/panel/api/clients/get/tgId/:tgId',
+        summary: 'Fetch clients by Telegram user ID. Returns an array since multiple clients can share the same Telegram ID.',
+        params: [
+          { name: 'tgId', in: 'path', type: 'integer', desc: 'Telegram user ID (numeric).' },
+        ],
+        response:
+          '{\n  "success": true,\n  "obj": [\n    {\n      "client": { "id": 1, "email": "alice@example.com", ... },\n      "inboundIds": [3, 5],\n      "externalLinks": [],\n      "usedTraffic": 1048576\n    }\n  ]\n}',
+      },
       {
         method: 'POST',
         path: '/panel/api/clients/add',