feat: Phase 4 grouping with entry-break dividers

Add group.py route, phase4.html template, and supporting state changes.
Photos are shown as a flat stream; clicking divider zones inserts
entry-break boundaries that split photos into labelled groups. Labels
persist via group_labels dict. Done materialises groups into state.groups
and advances to write phase.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/travel-memories/tests/fixtures/phase6_state.json

@@ -14,18 +14,20 @@
   "groups": [
     {
       "id": "g1", "photo_ids": ["asset-1"], "entry_type": "journal",
-      "title": "Arrival in Almaty", "body": "Chaos at the airport.",
+      "label": "", "title": "Arrival in Almaty", "body": "Chaos at the airport.",
       "location_city": "Almaty", "location_country": "Kazakhstan",
       "date": "2023-09-05", "hero_photo_id": "asset-1", "shortcode_hints": "",
       "status": "written"
     },
     {
       "id": "g2", "photo_ids": ["asset-2"], "entry_type": "story",
-      "title": "The Market", "body": "Colours everywhere.",
+      "label": "", "title": "The Market", "body": "Colours everywhere.",
       "location_city": "Almaty", "location_country": "Kazakhstan",
       "date": "2023-09-05", "hero_photo_id": "asset-2", "shortcode_hints": "gallery block",
       "status": "skipped"
     }
   ],
-  "notes": ""
+  "notes": "",
+  "dividers": [],
+  "group_labels": {}
 }