perf(clients): scale-audit remaining client/inbound endpoints to 200k

Drive every client/inbound/group endpoint at 100k-200k clients on PostgreSQL and fix the latent issues found in previously-unbenchmarked paths:

- enrichClientStats: chunk the email IN lookup (was an unchunked bind that crashed past 65535 clients without traffic rows, taking down GetInbounds/GetInboundDetail/GetAllInbounds)

- GetOnlineClients: add the missing nil-process guard its siblings already have, so ListPaged no longer panics before xray starts

- GetClientTrafficByEmail: read UUID/subId from the indexed clients table instead of parsing the inbound's full settings JSON (439ms to ~1.5ms, flat in N)

- BulkResetTraffic: replace the per-email serialized loop with one chunked bulk UPDATE in a single transaction

- DelDepleted: delegate to the already-batched BulkDelete instead of deleting each depleted client one by one

Adds a postgres-gated full endpoint sweep plus an A/B benchmark, and SQLite correctness tests for the changed methods.

frontend/public/openapi.json

@@ -1495,6 +1495,36 @@
         }
       }
     },
+    "/panel/api/server/getMigration": {
+      "get": {
+        "tags": [
+          "Server"
+        ],
+        "summary": "Stream a cross-engine migration file as an attachment: a .dump (SQL text) on SQLite, or a .db SQLite database built from the live data on PostgreSQL.",
+        "operationId": "get_panel_api_server_getMigration",
+        "responses": {
+          "200": {
+            "description": "Successful response",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "type": "object",
+                  "properties": {
+                    "success": {
+                      "type": "boolean"
+                    },
+                    "msg": {
+                      "type": "string"
+                    },
+                    "obj": {}
+                  }
+                }
+              }
+            }
+          }
+        }
+      }
+    },
     "/panel/api/server/getNewUUID": {
       "get": {
         "tags": [