feat: exclude bridgy fed content from search results
Add is_bridgyfed column to documents table. Mark at ingest time via
HTTP URL heuristic (only bridgy fed puts HTTP URLs in the site field).
Exclude from all search paths: keyword (turso + local SQLite),
semantic (local DB check in filter loop), and author browse queries.
Includes scripts/mark-bridgyfed for backfilling existing rows via
PLC directory resolution.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>