feat: ingestion-time content dedup + date filter UI
add content_hash (wyhash of title+content) to documents table. on
ingest, skip documents where the same author already has identical
content under a different rkey (cross-platform publishing dedup).
frontend: add date filter (any/week/month/year) with since param,
URL state sync, and active filter bar.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>