File Search: schema (FILE_SEARCH connector type, file_* tables, RLS) #1380

Part of #1379.

Schema change (opened before the migration, as CONTRIBUTING asks).

• ConnectorType.FILE_SEARCH.
• Tables: file_sources (setup state, schedule, lease, stats, summary), file_scans (setup dry runs), file_sync_runs, file_entries (one row per file: path, size, mtime, etag, sha256, status, attempts), file_chunks (text per PDF page or ~4,000-character segment, generated tsvector('simple')), file_shares (temporary links, token stored as hash only).
• Raw SQL: GIN on the tsvector, pg_trgm GIN on paths, a NOLOGIN role amcp_files with rights on file_* only, org_isolation RLS policies. File queries run in tenantTx + SET LOCAL ROLE amcp_files, so RLS applies even while the app role is a superuser.
• All rows cascade on connector delete.

Cloud and self-hosted: identical schema; additive migration, nothing changes for existing connectors.