feat: comprehensive test infrastructure improvements

- Fix unit test imports: add env setup in conftest.py before module imports
- Add 24 new auth router tests (RBAC, preferences, password validation)
- Add 16 new tests for litellm and chat_summary routers
- Apply black formatting and ruff linting across codebase
- Add pre-commit hooks configuration (black, ruff, file checks)
- Increase CI coverage threshold from 40% to 50%

Test Results:
- 206 tests passing (91 unit + 115 integration)
- Coverage: 58.79% on core modules
- auth.py: 57% → 85%, litellm.py: 23% → 87%, chat_summary.py: 41% → 100%
- auth_service: 96.51%, config: 100%, rbac: 93.48%

dashboard/backend/routers/chat_summary.py

@@ -1,18 +1,22 @@
-from fastapi import APIRouter, HTTPException
 import aiosqlite
+from fastapi import APIRouter, HTTPException
+
 from config import CHAT_SUMMARY_DB
 
 router = APIRouter()
 
+
 async def _db():
     return aiosqlite.connect(CHAT_SUMMARY_DB)
 
+
 @router.get("/dates")
 async def list_dates():
     async with await _db() as db:
         cursor = await db.execute("SELECT date FROM summaries ORDER BY date DESC")
         return [r[0] for r in await cursor.fetchall()]
 
+
 @router.get("/summary/{date}")
 async def get_summary(date: str):
     async with await _db() as db:
@@ -22,18 +26,21 @@ async def get_summary(date: str):
         raise HTTPException(404, "No summary for this date")
     return {"date": date, "content": row[0], "created_at": row[1]}
 
+
 @router.get("/messages/{date}")
 async def get_messages(date: str):
     async with await _db() as db:
         cursor = await db.execute(
             "SELECT group_name, sender_name, text, timestamp FROM messages WHERE date(timestamp)=? ORDER BY timestamp",
-            (date,)
+            (date,),
         )
         return [{"group": r[0], "sender": r[1], "text": r[2], "time": r[3]} for r in await cursor.fetchall()]
 
+
 @router.post("/trigger")
 async def trigger_summary():
     import os
+
     trigger_path = os.environ.get("CHAT_SUMMARY_TRIGGER", "/app/data/chat-summarizer/trigger")
     os.makedirs(os.path.dirname(trigger_path), exist_ok=True)
     with open(trigger_path, "w") as f: