Store how many bytes of each entry's artifacts are already on disk from
an earlier entry (content-addressed blob deduplication means shared
blobs are only stored once).
Design
------
- Add `cached_bytes INTEGER NOT NULL DEFAULT 0` to `archived_entries`
- Precompute at capture time via `database::refresh_entry_cached_bytes`
called after all artifacts are saved for every capture path
(web page, generic URL, tweet, yt-dlp/local)
- One-time migration in `initialize_schema`: detects missing column via
PRAGMA table_info, ALTERs the table, then back-fills all existing rows
with the correlated subquery
- `database::cascade_cached_bytes_after_delete` ready for when entry
deletion is implemented; designed to run asynchronously after the
delete is acknowledged to the user
- `cached_bytes` included in `EntrySummary` and all four SELECT paths
(list_root_entries, search_entries, list_entries_for_collection,
entries_for_tag) via the shared ENTRY_SELECT_COLS constant
Frontend
--------
- `EntryRow` shows a `% cached` sub-line under the size when non-zero,
with a tooltip showing the raw cached byte count
- No separate API endpoint or extra fetch — value rides in the existing
entries list response at zero extra query cost per read
