feat(m4): crop_fill setting + tape-line prototype (#99)

Add global crop_fill percent (default 90, 1-100) as a settings key with
FastAPI 4xx validation. Wires the value through to the inventory PNG
encode (non-retro: stored PNGs untouched). Adds a throwaway
TapePrototype.svelte behind ?tape-prototype for the #99 design sandbox.

Gates: check_requirements PASS, check_coverage PASS, settings tests 8/8.

backend/tests/test_settings_api.py

@@ -138,3 +138,51 @@ def test_settings_decrypt_failure_returns_reenter_state_not_500(monkeypatch):
         body = resp.json()
         assert body["inference_api_key"] == {"configured": False}
         db_mod._reset_for_tests()
+
+
+def test_settings_crop_fill_defaults_to_90(monkeypatch):
+    """Given: no crop_fill row; When: GET; Then: crop_fill == 90 (#99 req.4 default 90%)."""
+    with tempfile.TemporaryDirectory() as tmp:
+        _setup_env(monkeypatch, tmp)
+        from backend.app import app
+        client = TestClient(app)
+        db_mod.init_db()
+        resp = client.get("/api/v1/settings")
+        assert resp.status_code == 200
+        assert resp.json()["crop_fill"] == 90
+        db_mod._reset_for_tests()
+
+
+def test_settings_crop_fill_round_trips_as_plain_number(monkeypatch):
+    """Given: crop_fill in PUT; When: PUT then GET; Then: plain TEXT row (no value_enc) and a number back (#99 req.4)."""
+    with tempfile.TemporaryDirectory() as tmp:
+        _setup_env(monkeypatch, tmp)
+        from backend.app import app
+        client = TestClient(app)
+        db_mod.init_db()
+        resp = client.put("/api/v1/settings", json={"crop_fill": 75})
+        assert resp.status_code == 200
+        assert resp.json()["crop_fill"] == 75
+        conn = db_mod.get_connection()
+        row = conn.execute("SELECT value, value_enc FROM settings WHERE key='crop_fill'").fetchone()
+        assert row["value"] == "75" and row["value_enc"] is None
+        assert client.get("/api/v1/settings").json()["crop_fill"] == 75
+        db_mod._reset_for_tests()
+
+
+def test_settings_crop_fill_out_of_range_is_4xx_and_not_written(monkeypatch):
+    """Given: bad crop_fill; When: PUT; Then: 400 validation, nothing written (#99; rules in the 4xx layer)."""
+    with tempfile.TemporaryDirectory() as tmp:
+        _setup_env(monkeypatch, tmp)
+        from backend.app import app
+        client = TestClient(app)
+        db_mod.init_db()
+        for bad in (0, 101, -5, "abc", None):
+            resp = client.put("/api/v1/settings", json={"crop_fill": bad})
+            assert resp.status_code == 400, f"{bad!r} should be rejected"
+            assert resp.json()["error"]["code"] == "validation"
+        assert client.get("/api/v1/settings").json()["crop_fill"] == 90  # untouched default
+        # boundaries are valid
+        assert client.put("/api/v1/settings", json={"crop_fill": 100}).status_code == 200
+        assert client.put("/api/v1/settings", json={"crop_fill": 1}).status_code == 200
+        db_mod._reset_for_tests()