feat(processing): built-in 'look' presets (Cinematic/Vivid/Cozy/Soft/Cool)

Seed five curated, read-only post-processing templates so a non-expert gets
instant good-looking output before discovering the filter pipeline. Each is an
opinionated chain of existing filters (auto-crop/saturation/contrast/colour-
temperature/temporal-blur) tuned for a use case (films, games, evening ambience,
low-flicker, crisp cool-white).

Mirrors the built-in-gradient pattern: adds is_builtin to PostprocessingTemplate,
seeds missing looks on store init (idempotent, additive — no migration), and
makes built-ins read-only (update/delete raise -> 400; clone to customise).
Surfaced via the existing template picker + is_builtin in the response/type.

7 unit tests (seeding, idempotency, read-only protection, round-trip); full
suite green (1926 passed). (A runtime intensity slider is a follow-up — it needs
a filter-chain parameterisation layer.)

server/tests/test_postprocessing_looks.py

@@ -0,0 +1,81 @@
+"""Tests for built-in curated 'look' postprocessing templates."""
+
+import pytest
+
+from ledgrab.core.filters.registry import FilterRegistry
+from ledgrab.storage.postprocessing_template import PostprocessingTemplate
+from ledgrab.storage.postprocessing_template_store import (
+    _BUILTIN_LOOKS,
+    PostprocessingTemplateStore,
+)
+
+
+def test_builtins_are_seeded(tmp_db):
+    store = PostprocessingTemplateStore(tmp_db)
+    for key in _BUILTIN_LOOKS:
+        tpl = store.get_template(f"pp_builtin_{key}")
+        assert tpl.is_builtin is True
+        assert tpl.filters  # non-empty chain
+
+
+def test_builtin_filters_use_registered_ids(tmp_db):
+    store = PostprocessingTemplateStore(tmp_db)
+    for key in _BUILTIN_LOOKS:
+        tpl = store.get_template(f"pp_builtin_{key}")
+        for fi in tpl.filters:
+            assert FilterRegistry.is_registered(fi.filter_id), fi.filter_id
+
+
+def test_seeding_is_idempotent(tmp_db):
+    PostprocessingTemplateStore(tmp_db)
+    store2 = PostprocessingTemplateStore(tmp_db)
+    ids = [t.id for t in store2.get_all_templates() if t.id.startswith("pp_builtin_")]
+    assert sorted(ids) == sorted(f"pp_builtin_{k}" for k in _BUILTIN_LOOKS)
+
+
+def test_builtin_update_is_blocked(tmp_db):
+    store = PostprocessingTemplateStore(tmp_db)
+    with pytest.raises(ValueError, match="read-only"):
+        store.update_template("pp_builtin_vivid", name="Hacked")
+
+
+def test_builtin_delete_is_blocked(tmp_db):
+    store = PostprocessingTemplateStore(tmp_db)
+    with pytest.raises(ValueError, match="cannot be deleted"):
+        store.delete_template("pp_builtin_vivid")
+
+
+def test_user_template_still_editable_and_deletable(tmp_db):
+    store = PostprocessingTemplateStore(tmp_db)
+    tpl = store.create_template("My Look", filters=[])
+    assert tpl.is_builtin is False
+    store.update_template(tpl.id, description="changed")
+    store.delete_template(tpl.id)
+    with pytest.raises(ValueError):
+        store.get_template(tpl.id)
+
+
+def test_is_builtin_round_trips_through_dict():
+    tpl = PostprocessingTemplate.from_dict(
+        {
+            "id": "pp_x",
+            "name": "x",
+            "filters": [],
+            "created_at": "2026-01-01T00:00:00+00:00",
+            "updated_at": "2026-01-01T00:00:00+00:00",
+            "is_builtin": True,
+        }
+    )
+    assert tpl.is_builtin is True
+    assert tpl.to_dict()["is_builtin"] is True
+    # legacy dict without the field defaults to False
+    legacy = PostprocessingTemplate.from_dict(
+        {
+            "id": "pp_y",
+            "name": "y",
+            "filters": [],
+            "created_at": "2026-01-01T00:00:00+00:00",
+            "updated_at": "2026-01-01T00:00:00+00:00",
+        }
+    )
+    assert legacy.is_builtin is False