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

crates/archivr-core/src/capture.rs

@@ -751,7 +751,7 @@ pub fn perform_capture(
                     let _ = fs::remove_dir_all(store_path.join("temp").join(&timestamp));
                 }
 
-                record_media_entry(
+                let entry = record_media_entry(
                     &conn,
                     store_path,
                     user_id,
@@ -765,6 +765,7 @@ pub fn perform_capture(
                     byte_size,
                     title_hint,
                 )?;
+                database::refresh_entry_cached_bytes(&conn, entry.id)?;
                 database::finish_archive_run(&conn, run.id)?;
                 return Ok(CaptureResult {
                     run_uid: run.run_uid.clone(),
@@ -904,6 +905,9 @@ pub fn perform_capture(
                     }
                 }
 
+                // 7. Store how many bytes this entry gets "for free" from earlier entries.
+                database::refresh_entry_cached_bytes(&conn, entry.id)?;
+
                 database::finish_archive_run(&conn, run.id)?;
                 return Ok(CaptureResult {
                     run_uid: run.run_uid.clone(),
@@ -942,7 +946,7 @@ pub fn perform_capture(
             &timestamp,
         ) {
             Ok(_) => {
-                record_tweet_entry(
+                let tweet_entry = record_tweet_entry(
                     &conn,
                     store_path,
                     user_id,
@@ -952,6 +956,7 @@ pub fn perform_capture(
                     source,
                     &tweet_id,
                 )?;
+                database::refresh_entry_cached_bytes(&conn, tweet_entry.id)?;
                 database::finish_archive_run(&conn, run.id)?;
                 return Ok(CaptureResult {
                     run_uid: run.run_uid.clone(),
@@ -1042,7 +1047,7 @@ pub fn perform_capture(
         let _ = fs::remove_dir_all(store_path.join("temp").join(&timestamp));
     }
 
-    record_media_entry(
+    let media_entry = record_media_entry(
         &conn,
         store_path,
         user_id,
@@ -1056,6 +1061,7 @@ pub fn perform_capture(
         byte_size,
         None,  // title — populated in a later task
     )?;
+    database::refresh_entry_cached_bytes(&conn, media_entry.id)?;
     database::finish_archive_run(&conn, run.id)?;
 
     Ok(CaptureResult {