perf: hourly update check only with a browser open; opt-in retention, incremental auto-vacuum and Compact

- update-scheduler: default interval 1 h (DASHBOARD_UPDATE_CHECK_INTERVAL_MS and its 60 s floor kept);
  a scheduled tick with no /ws client is skipped, and the next browser to connect gets one catch-up
  check when the last one is stale (websocket.js onConnection). Manual check unchanged.
- db-retention.js: shared session purge (used by POST /api/settings/cleanup too), opt-in
  AGENTHUB_RETENTION_DAYS applied by the hourly maintenance in 25-session transactions with yields
  (max 5000/run), bounded incremental_vacuum when the freelist is large, and a manual Compact
  (VACUUM in a worker, converting to auto_vacuum=INCREMENTAL).
- db.js: new databases are INCREMENTAL; an existing one up to 32 MB is converted at startup.
- GET/PUT /api/settings/retention, POST /api/settings/compact (confirm: true); Settings → Data →
  Session cleanup gets the Automatic cleanup control and Compact now (en + vi).

client/src/pages/__tests__/Settings.test.tsx

@@ -77,6 +77,18 @@ vi.mock("../../lib/api", async (importOriginal) => {
             env_owned: { dir: false, hour: false, keep: false },
           })),
         },
+        retention: {
+          get: vi.fn(async () => ({
+            days: 0,
+            max_days: 3650,
+            env_owned: { days: false },
+            last_run: null,
+            space: null,
+            db_size: 0,
+            compacting: false,
+            last_compact: null,
+          })),
+        },
       },
     },
   };

client/src/pages/__tests__/__snapshots__/screens.snapshot.test.tsx.snap

@@ -9357,6 +9357,108 @@ exit 0
         </svg>
         Run Cleanup
       </button>
+      <div
+        class="border-t border-border pt-4 space-y-3"
+      >
+        <div>
+          <h4
+            class="text-xs font-medium text-fg-secondary"
+          >
+            Automatic cleanup
+          </h4>
+          <p
+            class="text-xs text-fg-muted"
+          >
+            Off by default. When on, the hourly maintenance deletes finished sessions (completed, error, abandoned) that started more than this many days ago — with their events, agents and token usage — a little at a time.
+          </p>
+        </div>
+        <div
+          class="flex flex-wrap items-end gap-3"
+        >
+          <div
+            class="bg-surface-2 rounded-lg px-4 py-3"
+          >
+            <label
+              class="text-xs text-fg-secondary block mb-2"
+              for="retention-days"
+            >
+              Delete sessions older than
+            </label>
+            <div
+              class="flex items-center gap-2"
+            >
+              <input
+                class="input w-20 text-sm text-right font-mono"
+                id="retention-days"
+                inputmode="numeric"
+                max="3650"
+                min="0"
+                step="1"
+                type="number"
+                value="0"
+              />
+              <span
+                class="text-xs text-fg-muted"
+              >
+                days
+              </span>
+            </div>
+          </div>
+          <button
+            class="btn-primary px-4 py-2 text-sm disabled:opacity-50 disabled:cursor-not-allowed"
+            disabled=""
+            type="button"
+          >
+            Save
+          </button>
+        </div>
+        <p
+          class="text-xs text-fg-secondary"
+        >
+          Automatic cleanup is off.
+        </p>
+        <div
+          class="flex flex-wrap items-center gap-3"
+        >
+          <button
+            class="text-xs px-3 py-1.5 rounded-md transition-colors disabled:opacity-50 text-fg-secondary hover:text-fg-primary hover:bg-surface-4 border border-border"
+            type="button"
+          >
+            <svg
+              class="lucide lucide-minimize2 w-3 h-3 inline mr-1"
+              fill="none"
+              height="24"
+              stroke="currentColor"
+              stroke-linecap="round"
+              stroke-linejoin="round"
+              stroke-width="2"
+              viewBox="0 0 24 24"
+              width="24"
+              xmlns="http://www.w3.org/2000/svg"
+            >
+              <polyline
+                points="4 14 10 14 10 20"
+              />
+              <polyline
+                points="20 10 14 10 14 4"
+              />
+              <line
+                x1="14"
+                x2="21"
+                y1="10"
+                y2="3"
+              />
+              <line
+                x1="3"
+                x2="10"
+                y1="21"
+                y2="14"
+              />
+            </svg>
+            Compact now
+          </button>
+        </div>
+      </div>
     </section>
     <section
       class="scroll-mt-24"

client/src/pages/__tests__/screens.snapshot.test.tsx

@@ -362,6 +362,18 @@ vi.mock("../../lib/api", async (importOriginal) => {
             env_owned: { dir: false, hour: false, keep: false },
           }),
         },
+        retention: {
+          get: r({
+            days: 0,
+            max_days: 3650,
+            env_owned: { days: false },
+            last_run: null,
+            space: null,
+            db_size: 0,
+            compacting: false,
+            last_compact: null,
+          }),
+        },
         exportData: () => "/api/settings/export",
         cleanup: r({
           ok: true,