Use parsed event detection in data lake as coverage signal

- sync-sources now runs a parallel PowerQuery checking for event.type
  population per source; count stored in new active_sources.parser_detected
- Coverage map marks a source as covered if parser_detected > 0, even
  without a matching local parser file (handles built-in/cloud parsers)
- UI parser cell shows "Parsed (N typed events detected)" for data-lake-
  detected parsers vs named local parser files
- Runtime ALTER TABLE migration adds parser_detected column to existing DBs

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

backend/db.py

@@ -36,6 +36,7 @@ class ActiveSource(Base):
     source_name = Column(String, unique=True, index=True)
     event_count = Column(Integer, default=0)
     synced_at = Column(DateTime, default=datetime.utcnow)
+    parser_detected = Column(Integer, default=0)  # >0 means parsed events seen in data lake
 
 
 class IngestSnapshot(Base):