refactor(routes): move memory domain into routes/memory/ subpackage (#5007)

Slice 2c of the route-domain reorganization (#4082/#4071, per
specs/architecture-runtime-inventory.md §6.3). Moves memory_routes.py into
routes/memory/, leaving a backward-compat sys.modules shim at the old path.
Pure file reorganization, no behavior change.

The shim uses sys.modules replacement (same pattern as the merged gallery
#4903 and research #4975 slices) so that `import routes.memory_routes`,
`from routes.memory_routes import X`, `importlib.import_module(...)`, and
the `import ... as mr` + `monkeypatch.setattr(mr, ...)` pattern used by
test_memory_routes_session_owner.py / test_memory_owner_isolation.py all
operate on the same module object the application uses.

The canonical module does NOT depend on the shim — routes/memory/
memory_routes.py imports only from services/, core/, src/, and stdlib (zero
internal routes/ coupling).

Four source-introspection test sites repointed to the new canonical path:
- test_direct_upload_limits.py
- test_upload_limits_centralized.py (two dict keys)
- test_vision_owner_scope.py

Adds tests/test_memory_routes_shim.py to pin the sys.modules shim contract
(legacy and canonical paths resolve to the same module object; monkeypatch
via legacy alias reaches the canonical module).

Verified: compileall clean; full suite 4219 passed, 3 skipped.

app.py

@@ -604,7 +604,7 @@ from routes.admin_wipe_routes import setup_admin_wipe_routes
 app.include_router(setup_admin_wipe_routes(session_manager))
 
 # Memory
-from routes.memory_routes import setup_memory_routes
+from routes.memory.memory_routes import setup_memory_routes
 memory_router = setup_memory_routes(memory_manager, session_manager, memory_vector=memory_vector)
 app.include_router(memory_router)
 from routes.skills_routes import setup_skills_routes