feat: squash merge all changes from develop-0.1.75

Squash of 124 commits from the legacy develop branch (develop-0.1.75)
onto a clean v0.1.75 upstream base, to simplify future upstream merges.

Key changes included:
- Refactor scope-level rate limiting to model-level rate limiting
- Antigravity gateway service improvements (smart retry, error policy)
- Digest session store (flat cache replacing Trie-based store)
- Client disconnect detection during streaming
- Gemini messages compatibility service enhancements
- Scheduler shuffle for thundering herd prevention
- Session hash generation improvements
- Frontend customizations (WeChat service, HomeView, etc.)
- Ops monitoring scope cleanup

.github/workflows/backend-ci.yml

@@ -17,6 +17,7 @@ jobs:
           go-version-file: backend/go.mod
           check-latest: false
           cache: true
+          cache-dependency-path: backend/go.sum
       - name: Verify Go version
         run: |
           go version | grep -q 'go1.25.7'
@@ -36,6 +37,7 @@ jobs:
           go-version-file: backend/go.mod
           check-latest: false
           cache: true
+          cache-dependency-path: backend/go.sum
       - name: Verify Go version
         run: |
           go version | grep -q 'go1.25.7'