refactor!: CQRS repository split — v0.3.0

FederationRepository (34 methods) → 4 focused traits:
  ActivityRepository  (2)  — idempotency tracking
  FollowRepository    (18) — follower/following graph + migration
  ActorRepository     (6)  — keypairs, remote actor cache, announce tracking
  BlocklistRepository (8)  — domain + actor blocklists

ApObjectHandler (10 methods) → 2 traits:
  ApContentReader  (3) — get_local_objects_for_user/page, count_local_posts
  ApObjectHandler  (9) — all inbox callbacks (on_create, on_mention, etc.)

Builder changes from positional args to named setters:
  ActivityPubService::builder(base_url)
    .activity_repo(arc)
    .follow_repo(arc)
    .actor_repo(arc)
    .blocklist_repo(arc)
    .user_repo(arc)
    .content_reader(arc)
    .object_handler(arc)
    .build()

No behaviour changes.

src/repository/activity.rs

@@ -0,0 +1,11 @@
+use anyhow::Result;
+use async_trait::async_trait;
+
+/// Tracks which inbound AP activity IDs have already been processed.
+/// Prevents duplicate handling when remote servers retry delivery.
+/// Implementations should enforce a UNIQUE constraint on stored IDs.
+#[async_trait]
+pub trait ActivityRepository: Send + Sync {
+    async fn is_activity_processed(&self, activity_id: &str) -> Result<bool>;
+    async fn mark_activity_processed(&self, activity_id: &str) -> Result<()>;
+}