feat: add translation billing stats and remove Export/Settings pages

- Add TranslationLog model to track translation API usage per task
- Integrate Dify API actual price (total_price) into translation stats
- Display translation statistics in admin dashboard with per-task costs
- Remove unused Export and Settings pages to simplify frontend
- Add GET /api/v2/admin/translation-stats endpoint

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/services/dify_client.py

@@ -40,6 +40,8 @@ class TranslationResponse:
     total_tokens: int
     latency: float
     conversation_id: str
+    total_price: float = 0.0
+    currency: str = "USD"
 
 
 @dataclass
@@ -50,6 +52,8 @@ class BatchTranslationResponse:
     latency: float
     conversation_id: str
     missing_markers: List[int] = field(default_factory=list)
+    total_price: float = 0.0
+    currency: str = "USD"
 
 
 class DifyTranslationError(Exception):
@@ -252,6 +256,11 @@ class DifyClient:
         translated_text = data.get("answer", "")
         usage = data.get("metadata", {}).get("usage", {})
 
+        # Extract price info from usage or metadata (may be string or number)
+        raw_price = usage.get("total_price", 0.0)
+        total_price = float(raw_price) if raw_price else 0.0
+        currency = usage.get("currency", "USD") or "USD"
+
         self._total_tokens += usage.get("total_tokens", 0)
         self._total_requests += 1
 
@@ -259,7 +268,9 @@ class DifyClient:
             translated_text=translated_text,
             total_tokens=usage.get("total_tokens", 0),
             latency=usage.get("latency", 0.0),
-            conversation_id=data.get("conversation_id", "")
+            conversation_id=data.get("conversation_id", ""),
+            total_price=total_price,
+            currency=currency
         )
 
     def translate_batch(
@@ -297,6 +308,11 @@ class DifyClient:
         answer = data.get("answer", "")
         usage = data.get("metadata", {}).get("usage", {})
 
+        # Extract price info from usage or metadata (may be string or number)
+        raw_price = usage.get("total_price", 0.0)
+        total_price = float(raw_price) if raw_price else 0.0
+        currency = usage.get("currency", "USD") or "USD"
+
         translations = self._parse_batch_response(answer, len(texts))
 
         # Check for missing markers
@@ -314,7 +330,9 @@ class DifyClient:
             total_tokens=usage.get("total_tokens", 0),
             latency=usage.get("latency", 0.0),
             conversation_id=data.get("conversation_id", ""),
-            missing_markers=missing_markers
+            missing_markers=missing_markers,
+            total_price=total_price,
+            currency=currency
         )
 
     def get_stats(self) -> dict: