feat(tweets): add re-archive button and fix thread-tweet orphan cleanup (#23)

Orphan cleanup bug: archiving x🧵A downloaded D/C/B/A JSONs and
media, but only registered artifacts for A. D/C/B files had no
entry_artifacts rows and were deleted as orphans.

Fix (staged scraper output, precise touched set):
- tweets::archive() stages all scraper output in temp/{ts}/tweet_stage/,
  validates, then renames JSONs to raw_tweets/. Return type changed from
  Result<bool> to Result<Vec<String>> (store-relative relpaths of every
  produced tweet JSON, i.e. the exact touched set).
- tweets::rearchive() (new): same staged approach but always runs the
  scraper. On scraper failure (tweet deleted/private), errors before
  touching raw_tweets/ so existing data is preserved.
- register_tweet_artifacts() (new private helper in capture.rs): registers
  every JSON in the touched set as a raw_tweet_json artifact, parses each
  for media blobs, registers those too. JSON read failure is a hard error
  with context, not a silent skip.
- record_tweet_entry() now accepts tweet_json_relpaths: &[String] and
  delegates artifact registration to register_tweet_artifacts().
- perform_capture() passes the returned vec from tweets::archive().

Re-archive feature:
- capture::perform_rearchive(): looks up entry by uid, validates
  tweet/tweet_thread, runs tweets::rearchive(), atomically swaps
  entry_artifacts in a DB transaction. archived_at, title, tags,
  collections are untouched.
- database: add get_entry_for_rearchive() and delete_entry_artifacts().
- POST /api/archives/:id/entries/:uid/rearchive: requires ROLE_USER,
  creates capture job, returns 202 + job_uid, runs perform_rearchive in
  spawn_blocking.
- Frontend: re-archive button in ContextRail for tweet/tweet_thread
  entries; polls job at 500ms; refreshes entry detail on success; shows
  error text on failure. Poll interval cleared before early-return on
  entry deselect to prevent stale updates.

frontend/src/styles.css

@@ -552,6 +552,24 @@ select {
   border-color: var(--accent);
 }
 
+.rail-rearchive-btn {
+  width: 100%;
+  padding: 6px 12px;
+  background: var(--surface-2, #2a2a2a);
+  color: var(--text, #e0e0e0);
+  border: 1px solid var(--border, #444);
+  border-radius: 4px;
+  cursor: pointer;
+  font-size: 13px;
+}
+.rail-rearchive-btn:hover:not(:disabled) {
+  background: var(--surface-3, #333);
+}
+.rail-rearchive-btn:disabled {
+  opacity: 0.6;
+  cursor: not-allowed;
+}
+
 /* ── Capture dialog ──────────────────────────────────────────────────────── */
 .capture-dialog {
   border: 1px solid var(--line);