Fix Copilot review: restore find-thread comments, batch evict

- threads.clj: restore find-thread with full comment pull (used by
  bot and curation). Add find-thread-meta for metadata-only queries.
- thread.clj: thread-resource uses find-thread-meta; readonly resource
  keeps find-thread with comments for bot/curation consumers.
- cache.clj: single-swap evict-expired! using reduce instead of
  per-key swap! to reduce contention and overhead.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kawasima

src/clj/back_channeling/boundary/threads.clj

@@ -7,6 +7,7 @@
   (pull [datomic id])
   (find-threads [datomic board-name q])
   (find-thread  [datomic thread-id])
+  (find-thread-meta [datomic thread-id])
   (find-watchers [datomic thread-id])
 
   (add-watcher    [datomic thread-id identity])
@@ -57,6 +58,17 @@
          vec))
 
   (find-thread [{:keys [connection]} thread-id]
+    (-> (d/pull (d/db connection)
+                '[:*
+                  {:thread/comments
+                   [:*
+                    {:comment/format [:db/ident]}
+                    {:comment/posted-by [:user/name :user/email]}]}]
+                thread-id)
+        (update-in [:thread/comments]
+                   (partial map-indexed #(assoc %2 :comment/no (inc %1))))))
+
+  (find-thread-meta [{:keys [connection]} thread-id]
     (d/pull (d/db connection)
             '[:db/id :thread/title :thread/since :thread/last-updated :thread/public?
               {:thread/watchers [:user/name :user/email]}]

src/clj/back_channeling/database/cache.clj

@@ -6,10 +6,13 @@
 (defrecord Boundary [cache scheduler])
 
 (defn- evict-expired!
-  "Force eviction of expired entries by touching every key via has?."
+  "Force eviction of all expired entries in a single atomic swap."
   [cache-atom]
-  (doseq [k (keys @cache-atom)]
-    (swap! cache-atom #(if (cache/has? % k) % (cache/evict % k)))))
+  (swap! cache-atom
+    (fn [c]
+      (reduce (fn [acc k]
+                (if (cache/has? acc k) acc (cache/evict acc k)))
+              c (keys c)))))
 
 (defmethod ig/init-key :back-channeling.database/cache [_ {:keys [ttl] :or {ttl (* 30 60 1000)}}]
   (let [cache-atom (atom (cache/ttl-cache-factory {} :ttl ttl))

src/clj/back_channeling/resource/thread.clj

@@ -61,8 +61,9 @@
    :handle-created (fn [_]
                      {:status "ok"})
    :handle-ok (fn [_]
-                (threads/find-thread datomic thread-id))))
+                (threads/find-thread-meta datomic thread-id))))
 
+;; Readonly resource returns full thread with comments (used by bot, curation)
 (defn thread-readonly-resource [{:keys [datomic]} thread-id]
   (liberator/resource base-resource
    :allowed-methods [:get]