refactor(output-targets): registry + coverage assertion for response builders

``_target_to_response`` in ``api/routes/output_targets.py`` used to be
an isinstance ladder over the three OutputTarget subclasses with a
silent fallback that fabricated a ``LedOutputTargetResponse`` for
unknown types (audit finding H3). The fallback masked exactly the
kind of bug we hit on the CSS side in Phase 1.1: a new target subclass
slipped past the ladder and got mis-shaped on the wire.

Replace the ladder with a ``_TARGET_RESPONSE_BUILDERS`` dict keyed by
the concrete subclass plus an import-time
``_assert_target_response_coverage()`` that requires the registry to
exactly match ``{WledOutputTarget, HALightOutputTarget,
Z2MLightOutputTarget}``. ``_target_to_response`` now raises
``RuntimeError`` instead of silently fabricating a LED response for an
unknown subclass — coverage is asserted at import so this branch is
unreachable in normal operation.

Tests: 5 new regression tests cover bijection between expected classes
and registered builders, callable shape, the rogue-target-raises
contract, and missing/extra entry rejection in the assertion. 24
existing output-target tests stay green; ruff clean.

server/src/ledgrab/api/routes/output_targets.py

@@ -175,26 +175,57 @@ def _validate_color_value_source(
         )
 
 
-def _target_to_response(target) -> OutputTargetResponse:
-    """Convert any OutputTarget to the appropriate typed response."""
-    if isinstance(target, WledOutputTarget):
-        return _led_target_to_response(target)
-    elif isinstance(target, HALightOutputTarget):
-        return _ha_light_target_to_response(target)
-    elif isinstance(target, Z2MLightOutputTarget):
-        return _z2m_light_target_to_response(target)
-    else:
-        # Fallback for unknown types — use LED response with defaults
-        return LedOutputTargetResponse(
-            id=target.id,
-            name=target.name,
-            description=target.description,
-            tags=target.tags,
-            created_at=target.created_at,
-            updated_at=target.updated_at,
+_TARGET_RESPONSE_BUILDERS: dict = {
+    WledOutputTarget: _led_target_to_response,
+    HALightOutputTarget: _ha_light_target_to_response,
+    Z2MLightOutputTarget: _z2m_light_target_to_response,
+}
+
+
+def _assert_target_response_coverage() -> None:
+    """Verify the response registry covers every concrete OutputTarget subclass.
+
+    Runs at module import. Surfaces a missing builder eagerly instead of
+    letting a request fall through to the previous silent fallback (which
+    used to return a defaults-filled LedOutputTargetResponse and quietly
+    misshape the payload for unknown target types).
+    """
+    expected = {WledOutputTarget, HALightOutputTarget, Z2MLightOutputTarget}
+    registered = set(_TARGET_RESPONSE_BUILDERS.keys())
+    missing = expected - registered
+    extra = registered - expected
+    if missing or extra:
+        problems = []
+        if missing:
+            problems.append(f"missing builders: {sorted(c.__name__ for c in missing)}")
+        if extra:
+            problems.append(f"unregistered classes: {sorted(c.__name__ for c in extra)}")
+        raise RuntimeError(
+            "_TARGET_RESPONSE_BUILDERS is out of sync with the OutputTarget "
+            "subclass set: " + "; ".join(problems)
         )
 
 
+_assert_target_response_coverage()
+
+
+def _target_to_response(target) -> OutputTargetResponse:
+    """Convert any OutputTarget to the appropriate typed response.
+
+    Dispatches via :data:`_TARGET_RESPONSE_BUILDERS` keyed by concrete
+    subclass. Raises ``RuntimeError`` for an unregistered subclass —
+    coverage is asserted at import, so this should never fire in
+    practice; if it does, the storage layer added a new OutputTarget
+    subclass without a matching response builder here.
+    """
+    builder = _TARGET_RESPONSE_BUILDERS.get(type(target))
+    if builder is None:
+        raise RuntimeError(
+            f"No response builder registered for OutputTarget subclass " f"{type(target).__name__}"
+        )
+    return builder(target)
+
+
 # ===== CRUD ENDPOINTS =====