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/archive.rs

@@ -28,6 +28,8 @@ pub struct EntrySummary {
     pub parent_entry_uid: Option<String>,
     /// True if a `favicon` artifact exists for this entry.
     pub has_favicon: bool,
+    /// Bytes of blobs already on disk from an earlier entry (precomputed at capture time).
+    pub cached_bytes: i64,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, serde::Serialize)]
@@ -206,7 +208,8 @@ pub fn list_root_entries(conn: &rusqlite::Connection, caller_bits: u32) -> Resul
             COUNT(ea.id) AS artifact_count,
             COALESCE(SUM(b.byte_size), 0) AS total_artifact_bytes,
             NULL AS parent_entry_uid,
-            EXISTS(SELECT 1 FROM entry_artifacts fav WHERE fav.entry_id = e.id AND fav.artifact_role = 'favicon') AS has_favicon
+            EXISTS(SELECT 1 FROM entry_artifacts fav WHERE fav.entry_id = e.id AND fav.artifact_role = 'favicon') AS has_favicon,
+            e.cached_bytes
          FROM archived_entries e
          JOIN source_identities si ON si.id = e.source_identity_id
          LEFT JOIN entry_artifacts ea ON ea.entry_id = e.id
@@ -238,6 +241,7 @@ pub fn list_root_entries(conn: &rusqlite::Connection, caller_bits: u32) -> Resul
                 total_artifact_bytes: row.get(8)?,
                 parent_entry_uid: row.get(9)?,
                 has_favicon: row.get::<_, i64>(10)? != 0,
+                cached_bytes: row.get(11)?,
             })
         })?
         .collect::<rusqlite::Result<Vec<_>>>()?;
@@ -425,6 +429,7 @@ pub fn list_entries_for_collection(
                 total_artifact_bytes: row.get(8)?,
                 parent_entry_uid: row.get(9)?,
                 has_favicon: row.get::<_, i64>(10)? != 0,
+                cached_bytes: row.get(11)?,
             })
         })?
         .collect::<rusqlite::Result<Vec<_>>>()?;
@@ -539,7 +544,8 @@ const ENTRY_SELECT_COLS: &str =
     e.visibility, si.canonical_url, COUNT(ea.id) AS artifact_count, \
     COALESCE(SUM(b.byte_size), 0) AS total_artifact_bytes, \
     parent.entry_uid AS parent_entry_uid, \
-    EXISTS(SELECT 1 FROM entry_artifacts fav WHERE fav.entry_id = e.id AND fav.artifact_role = 'favicon') AS has_favicon";
+    EXISTS(SELECT 1 FROM entry_artifacts fav WHERE fav.entry_id = e.id AND fav.artifact_role = 'favicon') AS has_favicon, \
+    e.cached_bytes";
 
 const ENTRY_FROM_JOINS: &str =
     "FROM archived_entries e \
@@ -649,6 +655,7 @@ pub fn search_entries(
                 total_artifact_bytes: row.get(8)?,
                 parent_entry_uid: row.get(9)?,
                 has_favicon: row.get::<_, i64>(10)? != 0,
+                cached_bytes: row.get(11)?,
             })
         })?
         .collect::<rusqlite::Result<Vec<_>>>()?;
@@ -810,7 +817,8 @@ pub fn entries_for_tag(
                 e.visibility, si.canonical_url, COUNT(ea.id) AS artifact_count,
                 COALESCE(SUM(b.byte_size), 0) AS total_artifact_bytes,
                 parent.entry_uid AS parent_entry_uid,
-                EXISTS(SELECT 1 FROM entry_artifacts fav WHERE fav.entry_id = e.id AND fav.artifact_role = 'favicon') AS has_favicon
+                EXISTS(SELECT 1 FROM entry_artifacts fav WHERE fav.entry_id = e.id AND fav.artifact_role = 'favicon') AS has_favicon,
+                e.cached_bytes
          FROM archived_entries e
          JOIN source_identities si ON si.id = e.source_identity_id
          LEFT JOIN entry_artifacts ea ON ea.entry_id = e.id
@@ -835,6 +843,7 @@ pub fn entries_for_tag(
                 total_artifact_bytes: row.get(8)?,
                 parent_entry_uid: row.get(9)?,
                 has_favicon: row.get::<_, i64>(10)? != 0,
+                cached_bytes: row.get(11)?,
             })
         })?
         .collect::<rusqlite::Result<Vec<_>>>()?;