feat(ap): ActivityPub spec compliance and profile completeness

Phase 1 — spec compliance:
- Add AS_PUBLIC constant; add to/cc fields to CreateActivity, DeleteActivity,
  UpdateActivity, AddActivity; populate on all broadcast call sites
- Add @context to outbox CreateActivity items
- Set manuallyApprovesFollowers: true to match actual Pending follow flow
- Gate PermissiveVerifier behind FEDERATION_DEBUG env var
- Add updated timestamp to Person actor JSON
- Improve actor update delivery logging

Phase 2a Batch 1 — AP layer:
- Add /inbox shared inbox route; add endpoints.sharedInbox to Person
- Paginate followers and following collections (20/page, OrderedCollectionPage)

Phase 2a Batch 2 — profile completeness:
- DB migrations: banner_path, also_known_as columns; user_profile_fields table
- ProfileField value object; UserProfileFieldsRepository port
- Banner image upload (stored via image-converter, surfaced as image in Person)
- alsoKnownAs field in Person (account migration support)
- Custom profile fields (up to 4 PropertyValue attachments in Person)
- Profile settings UI: banner preview/upload, alsoKnownAs input, fields form
- PUT /api/v1/profile/fields API endpoint

crates/application/src/use_cases/update_profile_fields.rs

@@ -0,0 +1,17 @@
+use domain::{
+    errors::DomainError,
+    events::DomainEvent,
+    value_objects::UserId,
+};
+
+use crate::{commands::UpdateProfileFieldsCommand, context::AppContext};
+
+pub async fn execute(ctx: &AppContext, cmd: UpdateProfileFieldsCommand) -> Result<(), DomainError> {
+    if cmd.fields.len() > 4 {
+        return Err(DomainError::ValidationError("Maximum 4 profile fields allowed".into()));
+    }
+    let user_id = UserId::from_uuid(cmd.user_id);
+    ctx.profile_fields_repository.set_fields(&user_id, cmd.fields).await?;
+    ctx.event_publisher.publish(&DomainEvent::UserUpdated { user_id }).await?;
+    Ok(())
+}