Split the adventures router into a package

`backend/app/routers/adventures.py` held 2353 lines and 35 endpoints. It is now
a package of 14 modules, the largest 443 lines.

The split moves text rather than rewriting it. An AST comparison against the
old file confirms all 86 definitions are identical, and the OpenAPI schema
still lists the same 35 operations.

Names a test replaces now live in `turns.py` only, and other modules reach them
as `turns.<name>`. Rebinding a re-exported alias changes the alias and leaves
every caller reading the original, so the package root does not re-export them.
A patch aimed at the old target raises `AttributeError` instead of passing while
doing nothing. Tests and the fixtures in `backend/tools/` say
`adventures.turns.<name>`.

The same rule keeps the turn lock working. One module owns `_active_turns`, so
one lock guards one set.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014Dix4oGV3njgWRdu7P9t6r

backend/tests/test_story_tree_baseline.py

@@ -80,7 +80,7 @@ def _make_world(monkeypatch, *, seeded_actions: int = 0):
     ScriptedProvider.replies = ["A reply."]
     ScriptedProvider.calls = 0
     ScriptedProvider.prompts = []
-    monkeypatch.setattr(adventures, "OpenAICompatibleProvider", ScriptedProvider)
+    monkeypatch.setattr(adventures.turns, "OpenAICompatibleProvider", ScriptedProvider)
     monkeypatch.setattr(auth, "resolve_provider_config", lambda s: auth.ProviderConfig(
         "http://fake", "k", "test-model", False))
     monkeypatch.setattr(limits, "rate_limit", lambda *a, **k: None)
@@ -102,7 +102,7 @@ def client(monkeypatch):
         yield c
     finally:
         app.dependency_overrides.clear()
-        adventures._active_turns.clear()
+        adventures.turns._active_turns.clear()
         Base.metadata.drop_all(bind=engine)
 
 
@@ -114,7 +114,7 @@ def long_client(monkeypatch):
         yield c
     finally:
         app.dependency_overrides.clear()
-        adventures._active_turns.clear()
+        adventures.turns._active_turns.clear()
         Base.metadata.drop_all(bind=engine)