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

backend/internal/handler/dto/types.go

@@ -2,11 +2,6 @@ package dto
 
 import "time"
 
-type ScopeRateLimitInfo struct {
-	ResetAt      time.Time `json:"reset_at"`
-	RemainingSec int64     `json:"remaining_sec"`
-}
-
 type User struct {
 	ID            int64     `json:"id"`
 	Email         string    `json:"email"`
@@ -126,9 +121,6 @@ type Account struct {
 	RateLimitResetAt *time.Time `json:"rate_limit_reset_at"`
 	OverloadUntil    *time.Time `json:"overload_until"`
 
-	// Antigravity scope 级限流状态（从 extra 提取）
-	ScopeRateLimits map[string]ScopeRateLimitInfo `json:"scope_rate_limits,omitempty"`
-
 	TempUnschedulableUntil  *time.Time `json:"temp_unschedulable_until"`
 	TempUnschedulableReason string     `json:"temp_unschedulable_reason"`