refactor(skin): replace trend with texture field on SkinConditionSnapshot

Remove the derived `trend` field (better computed from history by the MCP
agent) and add `texture: smooth|rough|flaky|bumpy` which LLM can reliably
assess from photos. Updates model, API, system prompt, tests, and frontend.

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

backend/tests/test_skincare.py

@@ -16,8 +16,8 @@ def test_create_snapshot_full(client):
         json={
             "snapshot_date": "2026-02-20",
             "overall_state": "good",
-            "trend": "improving",
             "skin_type": "combination",
+            "texture": "rough",
             "hydration_level": 3,
             "sebum_tzone": 4,
             "sebum_cheeks": 2,
@@ -32,7 +32,7 @@ def test_create_snapshot_full(client):
     assert r.status_code == 201
     data = r.json()
     assert data["overall_state"] == "good"
-    assert data["trend"] == "improving"
+    assert data["texture"] == "rough"
     assert "acne" in data["active_concerns"]
     assert data["hydration_level"] == 3