Server overhaul: shared helpers, dead code, synchronous tasks::start, stale comments

crate::lock replaces three poison-tolerant lock helpers; the rate limiter reuses auth::bearer_token; the landing page
reuses invites::display_code. Removed: AppError::not_implemented (D-004 placeholder, only its own test used it) and
the unused Mutation::mutation_id/entity_type. tasks::start never awaited or failed: now a plain fn returning AppState.
Milestone-ownership comments (M2a/M3/M4) rewritten to say what the code does.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TQd3pEP9UeN2uifbHknQ7B

server/crates/kvitt-server/src/wire/sync.rs

@@ -131,22 +131,6 @@ pub enum Mutation {
     Settlement(SettlementMutation),
 }
 
-impl Mutation {
-    pub fn mutation_id(&self) -> Uuid {
-        match self {
-            Self::Expense(m) => m.mutation_id,
-            Self::Settlement(m) => m.mutation_id,
-        }
-    }
-
-    pub fn entity_type(&self) -> EntityType {
-        match self {
-            Self::Expense(_) => EntityType::Expense,
-            Self::Settlement(_) => EntityType::Settlement,
-        }
-    }
-}
-
 /// E-32 `POST /sync` request: 1 to 100 mutations (more is 422 `batch_too_large`).
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize, ToSchema)]
 pub struct SyncPushRequest {