feat(value-sources): add sandboxed-Jinja template combinator

A new `template` value source evaluates a hardened, sandboxed Jinja
expression over the live values of other value sources — the system's
first float combinator.

Backend:
- Shared engine (utils/template_expr.py): ImmutableSandboxedEnvironment with
  filters/tests and auto-injected globals stripped; only min/max/abs/round/
  clamp exposed; rejects **, string/collection-literal repetition, attribute
  access and non-global calls; NaN/inf-safe result coercion.
- TemplateValueSource model + TemplateValueStream runtime: compile-once,
  primitives-only eval context, raw[name] exposure, eval_interval throttle,
  ref-counted input acquire/release, rename-safe hot-update.
- Validation: unbound-variable + reserved-name rejection, reference
  cycle/depth guards (depth-only at create, full cycle at update), runtime
  acquire() depth backstop, and delete referential-integrity.
- API: Create/Update/Response schemas + discriminated unions, _RESPONSE_MAP,
  and an advisory POST /value-sources/validate-template endpoint.
- Demo seed: a static source plus a template combinator example.

Frontend:
- Editor modal section: repeatable inputs list (EntitySelect rows), a
  zero-dependency Jinja syntax highlighter, a hints/reference panel, and a
  debounced live validator that gates Save (stale-response-safe).
- Graph editor: read-only template node with one edge per input.
- i18n (en/ru/zh), icon, and card rendering.

Tests: engine, stream, factory/cycle, validate endpoint, and demo seed.

server/tests/core/test_demo_seed.py

@@ -0,0 +1,50 @@
+"""Demo-seed regression tests (value sources, incl. the template combinator)."""
+
+from ledgrab.core.demo_seed import seed_demo_data
+from ledgrab.storage.database import Database
+from ledgrab.storage.value_source import StaticValueSource, TemplateValueSource
+from ledgrab.storage.value_source_store import ValueSourceStore
+
+
+def _seed(tmp_path):
+    db = Database(tmp_path / "demo.db")
+    seed_demo_data(db)
+    return db
+
+
+def test_demo_seeds_template_value_source(tmp_path):
+    db = _seed(tmp_path)
+    try:
+        store = ValueSourceStore(db)
+        by_id = {s.id: s for s in store.get_all_sources()}
+
+        base = by_id["vs_demo0001"]
+        boost = by_id["vs_demo0002"]
+        assert isinstance(base, StaticValueSource)
+        assert isinstance(boost, TemplateValueSource)
+        assert boost.template == "clamp(level * 1.5)"
+        assert boost.inputs == [{"name": "level", "value_source_id": "vs_demo0001"}]
+
+        # The reference graph is intact and consistent.
+        assert store.get_transitive_dependencies("vs_demo0002") == {"vs_demo0001"}
+        assert store.find_referencing_sources("vs_demo0001") == [boost.name]
+    finally:
+        db.close()
+
+
+def test_demo_template_evaluates_through_manager(tmp_path):
+    """The seeded template must actually evaluate over its seeded input."""
+    from ledgrab.core.processing.value_stream import ValueStreamManager
+
+    db = _seed(tmp_path)
+    try:
+        store = ValueSourceStore(db)
+        vsm = ValueStreamManager(value_source_store=store)
+        stream = vsm.acquire("vs_demo0002")
+        try:
+            # base level 0.5 -> clamp(0.5 * 1.5) = 0.75
+            assert abs(stream.get_value() - 0.75) < 1e-6
+        finally:
+            vsm.release("vs_demo0002")
+    finally:
+        db.close()