feat: precompute cached_bytes per entry in DB (#13)

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

frontend/src/App.jsx

@@ -241,7 +241,7 @@ export default function App() {
               </div>
             )}
             {view === 'archive' && (
-              <EntriesView
+            <EntriesView
                 entries={entries}
                 selectedEntryUid={selectedEntryUid}
                 onSelectEntry={selectEntry}

frontend/src/components/EntryRow.jsx

@@ -39,7 +39,14 @@ export default function EntryRow({ entry, archiveId, isSelected, onSelect }) {
       <div className="col-type">
         <span className="type-pill">{valueText(entry.entity_kind)}</span>
       </div>
-      <div className="col-size">{formatBytes(entry.total_artifact_bytes)}</div>
+      <div className="col-size">
+        <span className="size-total">{formatBytes(entry.total_artifact_bytes)}</span>
+        {entry.cached_bytes > 0 && entry.total_artifact_bytes > 0 && (
+          <span className="size-cached-pct" title={`${formatBytes(entry.cached_bytes)} already on disk from an earlier entry`}>
+            {Math.round(entry.cached_bytes / entry.total_artifact_bytes * 100)}% cached
+          </span>
+        )}
+      </div>
       <div className="url-cell col-url">{valueText(entry.original_url)}</div>
     </div>
   );

frontend/src/styles.css

@@ -310,7 +310,16 @@ select {
 .col-added { width: 162px; color: var(--muted); }
 .col-title { flex: 1 1 0; min-width: 0; overflow: hidden; display: flex; align-items: center; gap: 0.42em; }
 .col-type { width: 116px; }
-.col-size { width: 96px; }
+.col-size { width: 100px; display: flex; flex-direction: column; justify-content: center; gap: 1px; }
+.size-total { font-variant-numeric: tabular-nums; }
+.size-cached-pct {
+  font-size: 10px;
+  color: var(--muted);
+  opacity: 0.8;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
 .col-url { flex: 0 0 30%; min-width: 0; overflow: hidden; }
 
 .entry-header-row > div:first-child,