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/app/state.py

@@ -21,6 +21,7 @@ class Group:
     id: str
     photo_ids: list = field(default_factory=list)
     entry_type: str = "journal"   # journal | story
+    label: str = ""
     title: str = ""
     body: str = ""
     location_city: str = ""
@@ -42,6 +43,8 @@ class TripState:
     photos: list = field(default_factory=list)
     groups: list = field(default_factory=list)
     notes: str = ""
+    dividers: list = field(default_factory=list)    # [{"id": str, "after_order": int}]
+    group_labels: dict = field(default_factory=dict)  # {divider_id: label}
 
 
 def _state_path(album_id: str, app) -> Path: