feat: pool handlers + aiohttp routes

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

gates/handlers.py

@@ -0,0 +1,26 @@
+"""Pure request handlers — no aiohttp. Each returns the updated manifest dict."""
+from . import pool
+
+
+def handle_add(base, pool_id, data, ext, ts=0):
+    return pool.add_image(base, pool_id, data, ts=ts)
+
+
+def handle_remove(base, pool_id, index):
+    return pool.remove_slot(base, pool_id, index)
+
+
+def handle_active(base, pool_id, index):
+    return pool.set_active(base, pool_id, index)
+
+
+def handle_label(base, pool_id, index, label):
+    return pool.set_label(base, pool_id, index, label)
+
+
+def handle_list(base, pool_id):
+    return pool.read_manifest(base, pool_id)
+
+
+def handle_set_mask(base, pool_id, index, mask_png_bytes):
+    return pool.set_mask(base, pool_id, index, mask_png_bytes)  # Task 12