fix(correctness): pagination tiebreakers, dedup race, duration gating (0.87.6)

Three medium correctness fixes:

1. **Pagination ORDER BY missing tiebreakers** — four admin list queries
   sorted by a timestamp or `chapters.number` with no stable secondary
   sort. Add `id` tiebreakers across `repo::admin_view`,
   `repo::admin_audit`, `repo::bookmark`.

2. **`enqueue_pages` NOT EXISTS read-then-insert race** — concurrent
   admin clicks could land duplicate analyze_page jobs. Migration 0031
   adds a partial unique index mirroring 0014; query relies on
   `ON CONFLICT DO NOTHING`.

3. **`record_duration` overwrote a prior done row's duration on a
   non-terminal failed retry of a force-re-analyze.** Gate the call
   on "did this attempt actually write a row?".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/analysis/daemon.rs

@@ -313,6 +313,17 @@ impl WorkerContext {
             self.events.publish(event);
         }
 
+        // Stamp how long the vision dispatch took — but ONLY when a row
+        // was actually written this attempt. Otherwise a non-terminal
+        // failure on a force-re-analyze (page already has a `done` row)
+        // would overwrite the prior duration with the duration of a
+        // failed retry that wrote nothing new.
+        let wrote_row = match &result {
+            Ok(()) => true,
+            Err(_) if lease.attempts >= lease.max_attempts => true, // mark_failed wrote below
+            Err(_) => false,
+        };
+
         match result {
             Ok(()) => {
                 let _ = jobs::ack_done(&self.pool, lease.id).await;
@@ -341,10 +352,9 @@ impl WorkerContext {
             }
         }
 
-        // Stamp how long the vision dispatch took. Best-effort and ordered
-        // after the row is written (persist on success / mark_failed on
-        // terminal failure); a transient failure with no row updates nothing.
-        let _ = repo::page_analysis::record_duration(&self.pool, page_id, elapsed_ms).await;
+        if wrote_row {
+            let _ = repo::page_analysis::record_duration(&self.pool, page_id, elapsed_ms).await;
+        }
     }
 }