fix: SQLite database locked errors + add error status for 4xx/5xx

SQLite locking:
- Enable WAL journal mode in init_db (readers don't block writers)
- Set busy_timeout=30000ms in init_db
- Add timeout=30 to every aiosqlite.connect() across db.py, validator.py,
  enricher.py, main.py so connections wait up to 30s instead of crashing

Error status:
- 4xx/5xx HTTP responses are now prescreen_status='error' (server alive
  but broken/blocking) instead of 'live'
- Added 'error' counter to validator stats and orange Error stat box in UI
- Added ps-error CSS class (orange) and filter option in Browse tab

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/enricher.py

@@ -251,7 +251,7 @@ async def enrich_domain(domain: str) -> dict:
 
 
 async def save_enriched(data: dict):
-    async with aiosqlite.connect(SQLITE_PATH) as db:
+    async with aiosqlite.connect(SQLITE_PATH, timeout=30) as db:
         await db.execute(
             """INSERT INTO enriched_domains
                (domain, is_live, status_code, ssl_valid, ssl_expiry_days, cms,
@@ -284,7 +284,7 @@ async def save_enriched(data: dict):
 
 
 async def mark_job(domain: str, status: str, error: str = None):
-    async with aiosqlite.connect(SQLITE_PATH) as db:
+    async with aiosqlite.connect(SQLITE_PATH, timeout=30) as db:
         if status == "running":
             await db.execute(
                 "UPDATE job_queue SET status=?, started_at=datetime('now') WHERE domain=?",
@@ -315,7 +315,7 @@ async def worker_loop():
         if _paused:
             await asyncio.sleep(1)
             continue
-        async with aiosqlite.connect(SQLITE_PATH) as db:
+        async with aiosqlite.connect(SQLITE_PATH, timeout=30) as db:
             async with db.execute(
                 "SELECT domain FROM job_queue WHERE status='pending' LIMIT 100"
             ) as cur:
@@ -348,7 +348,7 @@ async def _assess_one(domain: str, language: str = "ES") -> None:
     except Exception as e:
         logger.error("AI: failed %s — %s", domain, e, exc_info=True)
         try:
-            async with aiosqlite.connect(SQLITE_PATH) as db:
+            async with aiosqlite.connect(SQLITE_PATH, timeout=30) as db:
                 await db.execute(
                     "UPDATE ai_queue SET status='failed', completed_at=datetime('now'), error=? WHERE domain=?",
                     (str(e)[:400], domain),
@@ -362,7 +362,7 @@ async def ai_worker_loop():
     logger.info("AI worker loop starting")
     # Reset any jobs left in 'running' state from a previous crashed worker
     try:
-        async with aiosqlite.connect(SQLITE_PATH) as db:
+        async with aiosqlite.connect(SQLITE_PATH, timeout=30) as db:
             result = await db.execute(
                 "UPDATE ai_queue SET status='pending' WHERE status='running'"
             )
@@ -375,7 +375,7 @@ async def ai_worker_loop():
     while True:
         rows = []
         try:
-            async with aiosqlite.connect(SQLITE_PATH) as db:
+            async with aiosqlite.connect(SQLITE_PATH, timeout=30) as db:
                 async with db.execute(
                     "SELECT domain, COALESCE(language,'ES') FROM ai_queue WHERE status='pending' LIMIT 5"
                 ) as cur: