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/db.py

@@ -101,7 +101,10 @@ _total_cache: int = 0
 
 
 async def init_db():
-    async with aiosqlite.connect(SQLITE_PATH) as db:
+    async with aiosqlite.connect(SQLITE_PATH, timeout=30) as db:
+        # WAL mode: concurrent reads don't block on writes; write lock held briefly
+        await db.execute("PRAGMA journal_mode=WAL")
+        await db.execute("PRAGMA busy_timeout=30000")
         await db.executescript(SCHEMA)
         # Run migrations (safe to re-run — silently skips existing columns)
         for sql in _MIGRATIONS:
@@ -237,7 +240,7 @@ async def get_domains(tld=None, page=1, limit=100, alpha_only=False, no_sld=Fals
         return total, []
 
     placeholders = ",".join("?" * len(domain_list))
-    async with aiosqlite.connect(SQLITE_PATH) as db:
+    async with aiosqlite.connect(SQLITE_PATH, timeout=30) as db:
         db.row_factory = aiosqlite.Row
         async with db.execute(
             f"SELECT * FROM enriched_domains WHERE domain IN ({placeholders})",
@@ -289,7 +292,7 @@ async def get_stats():
         loop = asyncio.get_event_loop()
         _total_cache, _tld_cache = await loop.run_in_executor(None, _tld_stats_sync)
 
-    async with aiosqlite.connect(SQLITE_PATH) as db:
+    async with aiosqlite.connect(SQLITE_PATH, timeout=30) as db:
         async with db.execute("SELECT COUNT(*) FROM enriched_domains") as cur:
             enriched = (await cur.fetchone())[0]
         threshold = int(os.getenv("SCORE_THRESHOLD", "60"))
@@ -351,7 +354,7 @@ async def get_enriched(min_score=0, cms=None, country=None, kit_digital=None,
         conditions.append("site_type = ?")
         params.append(site_type)
     where = "WHERE " + " AND ".join(conditions)
-    async with aiosqlite.connect(SQLITE_PATH) as db:
+    async with aiosqlite.connect(SQLITE_PATH, timeout=30) as db:
         db.row_factory = aiosqlite.Row
         async with db.execute(
             f"SELECT * FROM enriched_domains {where} ORDER BY score DESC LIMIT ? OFFSET ?",
@@ -366,7 +369,7 @@ async def get_enriched(min_score=0, cms=None, country=None, kit_digital=None,
 
 
 async def queue_ai(domains: list[str], language: str = "ES"):
-    async with aiosqlite.connect(SQLITE_PATH) as db:
+    async with aiosqlite.connect(SQLITE_PATH, timeout=30) as db:
         await db.executemany(
             """INSERT INTO ai_queue (domain, language) VALUES (?, ?)
                ON CONFLICT(domain) DO UPDATE SET language=excluded.language, status='pending'""",
@@ -376,7 +379,7 @@ async def queue_ai(domains: list[str], language: str = "ES"):
 
 
 async def get_ai_queue_status():
-    async with aiosqlite.connect(SQLITE_PATH) as db:
+    async with aiosqlite.connect(SQLITE_PATH, timeout=30) as db:
         async with db.execute("SELECT status, COUNT(*) FROM ai_queue GROUP BY status") as cur:
             rows = {r[0]: r[1] async for r in cur}
     return {
@@ -390,7 +393,7 @@ async def get_ai_queue_status():
 
 async def save_ai_assessment(domain: str, assessment: dict, site_analysis: dict = None):
     import json as _json
-    async with aiosqlite.connect(SQLITE_PATH) as db:
+    async with aiosqlite.connect(SQLITE_PATH, timeout=30) as db:
         # Upsert into enriched_domains (domain may not exist yet if assessed before full enrichment)
         await db.execute(
             """INSERT INTO enriched_domains (domain) VALUES (?) ON CONFLICT(domain) DO NOTHING""",
@@ -445,7 +448,7 @@ async def save_ai_assessment(domain: str, assessment: dict, site_analysis: dict
 
 async def save_prescreen_results(results: list[dict]):
     """Upsert prescreen HTTP results and/or DeepSeek niche/type classifications."""
-    async with aiosqlite.connect(SQLITE_PATH) as db:
+    async with aiosqlite.connect(SQLITE_PATH, timeout=30) as db:
         for r in results:
             domain = r.get("domain")
             if not domain:
@@ -477,7 +480,7 @@ async def save_prescreen_results(results: list[dict]):
 
 
 async def queue_domains(domains: list[str]):
-    async with aiosqlite.connect(SQLITE_PATH) as db:
+    async with aiosqlite.connect(SQLITE_PATH, timeout=30) as db:
         await db.executemany(
             "INSERT OR IGNORE INTO job_queue (domain) VALUES (?)",
             [(d,) for d in domains],
@@ -486,7 +489,7 @@ async def queue_domains(domains: list[str]):
 
 
 async def get_queue_status():
-    async with aiosqlite.connect(SQLITE_PATH) as db:
+    async with aiosqlite.connect(SQLITE_PATH, timeout=30) as db:
         async with db.execute("SELECT status, COUNT(*) FROM job_queue GROUP BY status") as cur:
             rows = {r[0]: r[1] async for r in cur}
     pending = rows.get("pending", 0)