E2: Migrate YouTube to declared rate-limit policy

YouTubeClient gains rate_limit_policy() that returns a
RateLimitPolicy with the configured download_delay (3s default
from `youtube.download_delay`). Engine reads this at
register_plugin time + applies to engine.worker.

set_engine still re-applies the delay so runtime reload_settings
updates flow through the same pathway. Other sources keep the
default policy (concurrency=1, delay=0) which matches their
current behavior — no migration needed beyond YouTube which is
the only source with a non-default download throttle today.

New pinning test asserts the policy shape (delay=3.0, concurrency=1).
Suite still green (2042 passed).

core/youtube_client.py

@@ -125,10 +125,24 @@ class YouTubeClient:
         # tests that bypass the orchestrator.
         self._engine = None
 
+    def rate_limit_policy(self):
+        """YouTube reads its download delay from user-tunable config
+        (``youtube.download_delay``, default 3s). Engine reads this
+        at ``register_plugin`` time, then ``set_engine`` runs and
+        re-applies if the config changed since instance construction."""
+        from core.download_engine import RateLimitPolicy
+        return RateLimitPolicy(
+            download_concurrency=1,
+            download_delay_seconds=float(self._download_delay),
+        )
+
     def set_engine(self, engine):
         """Engine callback — gives the client access to the central
         thread worker + state store. Engine calls this during
-        ``register_plugin`` if the plugin defines it."""
+        ``register_plugin`` if the plugin defines it. Worker delay
+        was already set from rate_limit_policy() — re-apply here so
+        runtime ``reload_settings`` updates take effect via the
+        same pathway."""
         self._engine = engine
         engine.worker.set_delay('youtube', float(self._download_delay))
 

tests/downloads/test_youtube_pinning.py

@@ -149,6 +149,17 @@ def test_set_engine_configures_worker_delay(yt_client_with_engine):
     assert engine.worker._get_delay('youtube') == 3.0
 
 
+def test_rate_limit_policy_reflects_configured_delay(yt_client_with_engine):
+    """Pinning (Phase E): YouTube's rate_limit_policy() returns a
+    RateLimitPolicy with the configured download_delay (3s default
+    from `youtube.download_delay` config). Engine reads this at
+    register_plugin time."""
+    client, _ = yt_client_with_engine
+    policy = client.rate_limit_policy()
+    assert policy.download_delay_seconds == 3.0
+    assert policy.download_concurrency == 1
+
+
 # ---------------------------------------------------------------------------
 # Query / cancel — engine-backed reads
 # ---------------------------------------------------------------------------