🛠️ chore(deps): upgrade better-auth to 1.6.15

Bump better-auth and its plugin packages from 1.4.6 → 1.6.15 on the
server. Lockstep with mobile-side bump (LOBE-10167).

Changes:
- better-auth: 1.4.6 → 1.6.15 (devDeps, deps, pnpm.overrides)
- @better-auth/expo / @better-auth/passkey: 1.4.6 → 1.6.15
- Drop the better-call 1.1.8 pin in three locations — better-auth 1.6.15 brings better-call 1.3.6 transitively, no override needed.

Verified locally:
- pnpm install + type-check clean
- vitest auth suite (define-config, route, email-whitelist, sso): 31/31 pass
- Drizzle migrations + curl smoke against /api/auth/{sign-up,sign-in,social,email-otp,passkey} endpoints
- End-to-end with mobile client against this server: email signup, email/password sign-in, email OTP, Google OAuth, session restore — all OK

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

package.json

@@ -166,8 +166,7 @@
     "@types/react": "19.2.13",
     "antd": "6.3.5",
     "baseline-browser-mapping": "2.10.31",
-    "better-auth": "1.4.6",
-    "better-call": "1.1.8",
+    "better-auth": "1.6.15",
     "drizzle-orm": "^0.45.1",
     "fast-xml-parser": "5.4.2",
     "lexical": "0.42.0",
@@ -185,8 +184,8 @@
     "@aws-sdk/s3-request-presigner": "~3.932.0",
     "@azure-rest/ai-inference": "1.0.0-beta.5",
     "@azure/core-auth": "^1.10.1",
-    "@better-auth/expo": "1.4.6",
-    "@better-auth/passkey": "1.4.6",
+    "@better-auth/expo": "1.6.15",
+    "@better-auth/passkey": "1.6.15",
     "@cfworker/json-schema": "^4.1.1",
     "@chat-adapter/discord": "^4.23.0",
     "@chat-adapter/slack": "^4.23.0",
@@ -331,8 +330,7 @@
     "antd-style": "4.1.0",
     "async-retry": "^1.3.3",
     "bcryptjs": "^3.0.3",
-    "better-auth": "1.4.6",
-    "better-call": "1.1.8",
+    "better-auth": "1.6.15",
     "brotli-wasm": "^3.0.1",
     "buffer.js": "npm:buffer@^6.0.3",
     "chat": "^4.23.0",
@@ -571,8 +569,6 @@
       "@vitest/coverage-v8": "3.2.4",
       "antd": "6.3.5",
       "baseline-browser-mapping": "2.10.31",
-      "better-auth": "1.4.6",
-      "better-call": "1.1.8",
       "drizzle-orm": "^0.45.1",
       "fast-xml-parser": "5.4.2",
       "lexical": "0.42.0",