feat: production hardening — security, scale, protocol, DX

Breaking changes to FederationRepository, ApObjectHandler, ApUser:

FederationRepository:
- add is_activity_processed / mark_activity_processed (inbox idempotency)
- add get_accepted_follower_inboxes (DB-side dedup/filtering, replaces in-memory load-all)

ApObjectHandler:
- add on_announce_of_remote (cross-server boosts, previously silently dropped)

ApUser:
- add manually_approves_followers: bool
- add actor_type: ApActorType (was hardcoded Person)

Security:
- block check before actor HTTP fetch in Follow (prevents SSRF on blocked actors)
- 4xx responses use generic "not found"/"bad request" (no internal leak)
- 1 MB DefaultBodyLimit on inbox routes
- zeroize private key after generation

Delivery:
- all broadcasts are now non-blocking (tokio::spawn fallback, or EventPublisher queue)
- EventPublisher redesigned with typed FederationEvent enum (DeliveryRequested/DeliveryFailed)
- new deliver_to_inbox() public method for queue consumers
- configurable delivery_max_attempts and delivery_initial_delay_secs via builder
- Follow saved as Pending BEFORE delivery (race condition fix)

Router:
- GET /users/{id} (actor), GET /users/{id}/followers, GET /users/{id}/following now mounted

Protocol:
- mention extraction from Create/Update tag arrays → on_mention() dispatched
- WebFinger: add aliases field (acct: URI + AP actor URL)
- outbox: add last link, use count_local_posts for totalItems
- idempotency guard added to every inbound activity receive()
- actor serializes display_name and configurable actor_type/manually_approves_followers

Bump: 0.1.10 → 0.2.0

src/error.rs

@@ -33,15 +33,18 @@ where
 impl axum::response::IntoResponse for Error {
     fn into_response(self) -> axum::response::Response {
         let status = self.1;
+        // Always log the real error internally; never expose it to the client.
         if status.is_server_error() {
             tracing::error!(error = %self.0, status = status.as_u16(), "federation error");
         } else {
-            tracing::debug!(error = %self.0, status = status.as_u16(), "federation response");
+            tracing::debug!(error = %self.0, status = status.as_u16(), "federation client error");
         }
-        let body = if status.is_server_error() {
-            "internal server error".to_string()
-        } else {
-            self.0.to_string()
+        let body = match status {
+            StatusCode::NOT_FOUND => "not found",
+            StatusCode::BAD_REQUEST => "bad request",
+            StatusCode::UNAUTHORIZED => "unauthorized",
+            StatusCode::FORBIDDEN => "forbidden",
+            _ => "internal server error",
         };
         (status, body).into_response()
     }