feat: profile/settings (rest timer toggle)

Settings infrastructure + one working preference:

- New user_settings table (JSON blob per user, so adding
  future keys needs no migration).
- db.get_settings / update_settings helpers (merge semantics).
- GET/PUT /api/settings endpoints.
- New Settings tab in the Mini App with a rest-timer on/off
  toggle. Setting is loaded on init and written through on
  change; the rest-timer display now respects it.

Units (kg/lb) and language are intentionally left unwired for
now — each needs end-to-end display/input changes and deserve
focused passes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_db.py

@@ -309,6 +309,40 @@ class TestEvents:
         assert result == -1
 
 
+# ── user settings ────────────────────────────────────────────────
+
+
+class TestSettings:
+    def test_default_empty(self, tmp_db):
+        assert db.get_settings(1) == {}
+
+    def test_update_creates(self, tmp_db):
+        result = db.update_settings(1, {"rest_timer": False})
+        assert result == {"rest_timer": False}
+        assert db.get_settings(1) == {"rest_timer": False}
+
+    def test_update_merges(self, tmp_db):
+        db.update_settings(1, {"rest_timer": False})
+        result = db.update_settings(1, {"units": "lb"})
+        assert result == {"rest_timer": False, "units": "lb"}
+
+    def test_update_overwrites_key(self, tmp_db):
+        db.update_settings(1, {"rest_timer": False})
+        db.update_settings(1, {"rest_timer": True})
+        assert db.get_settings(1)["rest_timer"] is True
+
+    def test_settings_are_per_user(self, tmp_db):
+        db.update_settings(1, {"rest_timer": False})
+        db.update_settings(2, {"rest_timer": True})
+        assert db.get_settings(1) == {"rest_timer": False}
+        assert db.get_settings(2) == {"rest_timer": True}
+
+    def test_patch_must_be_dict(self, tmp_db):
+        import pytest
+        with pytest.raises(TypeError):
+            db.update_settings(1, "not a dict")
+
+
 # ── update_workout ───────────────────────────────────────────────