feat(server/tts): upgrade dashscope-cosyvoice adapter to v2 two-step REST

DashScope dropped cosyvoice-v1 from its REST-supported model list. v2
(and v3+) speak a different shape: voice / format / sample_rate live
under `input`, not `parameters`; non-streaming responses return
`output.audio.url` (signed OSS URL) instead of inline `output.audio.data`
base64. The previous adapter sent v1-shaped bodies to a bare
`https://dashscope-intl.aliyuncs.com/api/v1` baseURL and parsed
`audio.data`, which 404'd before the migration and would 200-with-no-
audio after — both invisible regressions for the gateway.

Adapter changes:
- Rewrite request body to v2 schema (voice/format under input).
- Add follow-up GET against `output.audio.url`; stream into ArrayBuffer
  with a 25 MB hard cap and explicit drain-tracking finally, so a
  misbehaving URL cannot exhaust memory and a half-read body cannot
  hang a connection.
- Re-document baseURL contract: adapters do NOT append path; ops must
  configure the FULL endpoint URL (root cause of the original 404
  storm). DEFAULT_COSYVOICE_MODEL bumped to `cosyvoice-v2`, default
  voice to `longxiaochun_v2`.

Voice catalog: regenerated with 19 representative cosyvoice-v2 voices
(assistant / customer-service / child / en-US / en-GB / ja-JP / ko-KR)
so the frontend voice picker is no longer a 2-entry stub. Full catalog
(100+) remains on the Alibaba docs page — we'll sync on demand rather
than scrape.

Seed script: `--dashscope-region intl|cn` (default `intl`),
`--dashscope-upstream-model cosyvoice-v2`, baseURL now resolves to
`https://<host>/api/v1/services/audio/tts/SpeechSynthesizer` so a
mis-typed region or path cannot reintroduce the 404.

Tests: new dashscope-cosyvoice.test.ts covers v2 body shape (asserts
`parameters` absent — regression), audio.url follow-up fetch, 401
propagation with `.status`, empty-envelope falling back into the
router's recoverable-error path, and catalog freshness (no leftover v1
ids). Verified locally against the staging DashScope key: 200 +
playable mp3 end to end.

apps/server/scripts/seed-router-config.ts

@@ -49,6 +49,10 @@ interface Args {
   azureRegion: string
   azureTtsModel: string
   dashscopeTtsModel: string
+  /** `intl` → dashscope-intl.aliyuncs.com (Singapore); `cn` → dashscope.aliyuncs.com (Beijing). */
+  dashscopeRegion: string
+  /** Concrete cosyvoice variant the adapter calls upstream. Independent from `dashscopeTtsModel` (the gateway-facing alias). */
+  dashscopeUpstreamModel: string
   defaultTtsModel: string | undefined
 }
 
@@ -76,7 +80,9 @@ function parseArgs(argv: string[]): Args {
     defaultChatModel: values['default-chat-model'] ?? 'chat-default',
     azureRegion: values['azure-region'] ?? 'eastasia',
     azureTtsModel: values['azure-tts-model'] ?? 'microsoft/v1',
-    dashscopeTtsModel: values['dashscope-tts-model'] ?? 'alibaba/cosyvoice-v1',
+    dashscopeTtsModel: values['dashscope-tts-model'] ?? 'alibaba/cosyvoice-v2',
+    dashscopeRegion: values['dashscope-region'] ?? 'intl',
+    dashscopeUpstreamModel: values['dashscope-upstream-model'] ?? 'cosyvoice-v2',
     defaultTtsModel: values['default-tts-model'],
   }
 }
@@ -132,14 +138,21 @@ function buildDashscope(args: Args, plaintext: string, envelope: EnvelopeCrypto)
     modelName: args.dashscopeTtsModel,
     keyEntryId,
   })
+  // dashscope-cosyvoice adapter expects the FULL non-streaming endpoint path —
+  // it does not append `/services/audio/tts/SpeechSynthesizer` itself. A bare
+  // `/api/v1` baseURL was the root cause of the 404 storm during the v1→v2
+  // migration; do not regress here.
+  const host = args.dashscopeRegion === 'cn'
+    ? 'dashscope.aliyuncs.com'
+    : 'dashscope-intl.aliyuncs.com'
   return {
     ttsModelName: args.dashscopeTtsModel,
     ttsModel: {
       provider: 'dashscope-cosyvoice',
       upstreams: [{
-        baseURL: 'https://dashscope-intl.aliyuncs.com/api/v1',
+        baseURL: `https://${host}/api/v1/services/audio/tts/SpeechSynthesizer`,
         keys: [{ id: keyEntryId, ciphertext }],
-        adapterParams: {},
+        adapterParams: { model: args.dashscopeUpstreamModel },
       }],
     },
   }