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/validator.py

@@ -66,6 +66,7 @@ _val_stats: dict = {
     "processed":  0,
     "live":       0,
     "dead":       0,
+    "error":      0,
     "parked":     0,
     "redirect":   0,
     "skipped":    0,
@@ -148,6 +149,11 @@ async def _check_domain(domain: str) -> dict:
                         result["prescreen_status"] = "parked"
                         return result
 
+            # 4xx / 5xx = server is alive but site is broken/blocking
+            if resp.status_code >= 400:
+                result["prescreen_status"] = "error"
+                return result
+
             result["prescreen_status"] = "live"
             return result
 
@@ -201,7 +207,7 @@ async def _filter_unvalidated(domains: list[str], rescan_dead: bool = False) ->
         condition = "prescreen_status IS NOT NULL AND prescreen_status != 'dead'"
     else:
         condition = "prescreen_status IS NOT NULL"
-    async with aiosqlite.connect(SQLITE_PATH) as db:
+    async with aiosqlite.connect(SQLITE_PATH, timeout=30) as db:
         async with db.execute(
             f"SELECT domain FROM enriched_domains "
             f"WHERE domain IN ({placeholders}) AND {condition}",
@@ -212,7 +218,7 @@ async def _filter_unvalidated(domains: list[str], rescan_dead: bool = False) ->
 
 
 async def _save_batch(results: list[dict]):
-    async with aiosqlite.connect(SQLITE_PATH) as db:
+    async with aiosqlite.connect(SQLITE_PATH, timeout=30) as db:
         for r in results:
             await db.execute(
                 """INSERT INTO enriched_domains
@@ -319,7 +325,7 @@ def start_validator(tld_filter: Optional[str] = None, rescan_dead: bool = False)
     # domains, so restarting from 0 is safe and fast even for the same TLD.
     _val_stats.update(
         running=True,
-        processed=0, live=0, dead=0, parked=0,
+        processed=0, live=0, dead=0, error=0, parked=0,
         redirect=0, skipped=0, offset=0, rate=0.0,
         tld_filter=tld_filter,
         rescan_dead=rescan_dead,