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/test_phase4.py

@@ -0,0 +1,47 @@
+import json
+
+
+def test_photos_shown_as_stream(base_url, page, seed_state):
+    album_id = seed_state("phase4_state")
+    page.goto(f"{base_url}/group?album_id={album_id}")
+    assert page.locator(".stream-photo").count() == 2
+
+
+def test_insert_divider_creates_group_boundary(base_url, page, seed_state, flask_app):
+    album_id = seed_state("phase4_state")
+    page.goto(f"{base_url}/group?album_id={album_id}")
+    page.locator(".divider-zone").first.hover()
+    page.locator(".insert-divider-btn").first.click()
+    page.wait_for_timeout(300)
+    assert page.locator(".group-block").count() == 2
+
+
+def test_remove_divider_merges_groups(base_url, page, seed_state):
+    album_id = seed_state("phase4_state")
+    page.goto(f"{base_url}/group?album_id={album_id}")
+    page.locator(".divider-zone").first.hover()
+    page.locator(".insert-divider-btn").first.click()
+    page.wait_for_timeout(200)
+    page.locator(".remove-divider-btn").first.click()
+    page.wait_for_timeout(200)
+    assert page.locator(".group-block").count() == 1
+
+
+def test_label_edit_persists(base_url, page, seed_state, flask_app):
+    album_id = seed_state("phase4_state")
+    page.goto(f"{base_url}/group?album_id={album_id}")
+    page.locator(".divider-zone").first.hover()
+    page.locator(".insert-divider-btn").first.click()
+    page.wait_for_timeout(200)
+    page.locator(".group-label").first.fill("Morning walk")
+    page.locator(".group-label").first.press("Enter")
+    page.wait_for_timeout(300)
+    page.reload()
+    assert "Morning walk" in page.locator(".group-label").first.input_value()
+
+
+def test_done_advances_to_write(base_url, page, seed_state):
+    album_id = seed_state("phase4_state")
+    page.goto(f"{base_url}/group?album_id={album_id}")
+    page.locator("#done-btn").click()
+    page.wait_for_url("**/write**")