feat: add per-user retention settings to users.toml

New [user.retention] TOML sub-block with auto_close_days and
auto_delete_days. Both default to 0, which disables the respective
sweep — a conservative default that preserves current behaviour for
existing deployments and lets admins test auto-close in isolation.

Only parsing + the test User literals are touched here; the actual
lazy sweep consuming these values lands in the next commit.

server/users.toml.example

@@ -8,6 +8,13 @@ slug = "dr_mueller"
 api_key = "change-me-to-a-secure-key"
 web_password = "$2b$12$..."
 role = "doctor"
+  # Optional per-user retention. Both fields default to 0 (feature
+  # disabled). auto_close_days = 7 + auto_delete_days = 30 means cases
+  # without new recordings auto-close after 7 days and are irreversibly
+  # removed 30 days later. Changes require a server restart.
+  [user.retention]
+  auto_close_days = 7
+  auto_delete_days = 30
 
 [[user]]
 slug = "dr_schmidt"