refactor(routes): move contacts domain into routes/contacts/ subpackage (#5227)

Slice 2e of the route-domain reorganization (#4082/#4071, per
specs/architecture-runtime-inventory.md §6.3). Moves contacts_routes.py into
routes/contacts/, 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, research #4975, memory #5007, and history #5090 slices) so that
`import routes.contacts_routes`, `from routes.contacts_routes import X`,
`importlib.import_module(...)`, the string-targeted
`monkeypatch.setattr("routes.contacts_routes.SETTINGS_FILE", ...)` used by
test_carddav_password_encryption.py, and the `import ... as cr` +
`setattr(cr, ...)` pattern in test_contacts_add_null_name.py all operate on
the same module object the application uses. This also keeps the mutable
module state `_contact_cache` identical across import paths.

The canonical module does NOT depend on the shim — routes/contacts/
contacts_routes.py imports only from core/, src/, and stdlib (zero internal
routes/ coupling). The inbound edge from routes/email_helpers.py (imports
_fetch_contacts) keeps working through the shim.

Zero source-introspection landmines — no test reads this file by path.

Adds tests/test_contacts_routes_shim.py to pin the sys.modules shim contract
(same-object + string-targeted monkeypatch reach-through).

Verified: compileall clean; full suite 4485 passed, 3 skipped.

app.py

@@ -813,7 +813,7 @@ from routes.vault_routes import setup_vault_routes
 app.include_router(setup_vault_routes())
 
 # Contacts (CardDAV)
-from routes.contacts_routes import setup_contacts_routes
+from routes.contacts.contacts_routes import setup_contacts_routes
 app.include_router(setup_contacts_routes())
 
 from companion import setup_companion_routes