fast log clear + robust API error handling

- fix: "Log leeren" without account filter used a plain DELETE over
  millions of rows. That held the sqlite write lock long enough that
  the scheduler's own log writes started failing with
  "database is locked", and the request itself eventually 500'd back
  as HTML "Internal Server Error" — which the frontend then tried to
  JSON.parse, producing the popup the user reported. Now uses
  DROP+CREATE for full clears (near-instant) and wal_checkpoint to
  reclaim space; per-account clear still uses a normal DELETE.
- Raise sqlite busy_timeout from 10s to 60s so a legitimately slow
  writer no longer starves smaller ones.
- retry-failed: single bulk DELETE with mail_uid IN (...) per account
  instead of one query per uid.
- Both endpoints now return proper HTTPException(500, ...) with detail
  instead of letting the exception bubble as plain text.
- Frontend: parseResponse() reads text first, then tries JSON. Both
  clearLogs() and retryFailed() surface the actual server message
  instead of crashing on non-JSON responses.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/database.py

@@ -14,7 +14,7 @@ if settings.database_url.startswith("sqlite"):
     def _set_sqlite_pragmas(dbapi_connection, _):
         cursor = dbapi_connection.cursor()
         cursor.execute("PRAGMA journal_mode=WAL")
-        cursor.execute("PRAGMA busy_timeout=10000")
+        cursor.execute("PRAGMA busy_timeout=60000")
         cursor.execute("PRAGMA synchronous=NORMAL")
         cursor.close()