feat(data): update Store interface for source-aware queries
Update method signatures for InsertIRCLink, InsertQuote, and
InsertImage to accept struct pointers. Add SourceFilter parameter
to GetRecentIRCLinks, GetRecentImages, GetRecentQuotes,
SearchIRCLinks, SearchQuotes, and GetIRCLinksByURL. Add
applySourceFilter helper to GormStore. Update all callers in
handlers, scheduler, and tests. Fix composite index names to be
unique per table (idx_link_source, idx_image_source,
idx_quote_source) to avoid SQLite global index collisions.
Add source_filter_test.go for SourceFilter.IsEmpty tests.