Phase 2c: remove audience + PostPush + PostNotification + AudienceRequest/Response

v0.6.2 wire fork: every persona-identifying direct push is gone. Public posts
propagate only through the CDN (pull + header-diff neighbor propagation).
Encrypted posts propagate only through pull with merged author-or-recipient
match. There is no remaining sender→recipient traffic correlation signal on
the wire for content.

Protocol (network-breaking):
- Retire MessageType 0x42 (PostNotification), 0x43 (PostPush),
  0x44 (AudienceRequest), 0x45 (AudienceResponse). Their payload structs are
  deleted along with the handlers and senders.
- SocialDisconnectNotice (0x71) / SocialAddressUpdate (0x70) sender
  functions targeting audience are deleted; the existing handlers stay
  (both already dead code on the send side).

Core removals:
- `push_to_audience`, `notify_post`, `push_delete`,
  `push_disconnect_to_audience`, `push_address_update_to_audience`,
  `send_audience_request`, `send_audience_response`, `send_to_audience` —
  all gone from network.rs.
- `handle_post_notification` removed from connection.rs.
- `request_audience`, `approve_audience`, `deny_audience`,
  `remove_audience`, `list_audience_members`, `list_audience` removed from
  Node.
- `audience_pushed` step removed from post creation.
- `AudienceDirection`, `AudienceStatus`, `AudienceRecord`,
  `AudienceApprovalMode` removed from types.
- Storage: `store_audience`, `list_audience`, `list_audience_members`,
  `remove_audience`, `row_to_audience_record`, `audience_crud` test, the
  `audience` CREATE TABLE, and the audience-dependent social route rebuild
  branch all removed. Upgraded DBs retain the orphan `audience` table;
  nothing touches it.

Follow-on cleanups:
- `SocialRelation::Audience` + `::Mutual` collapsed into just `Follow`.
  The Display/FromStr impl accepts legacy "audience"/"mutual" strings from
  pre-v0.6.2 DBs and maps them to Follow.
- Blob-eviction priority function drops the audience factor; relationship
  is now own-author vs followed vs other. Tests updated accordingly.
- `CommentPermission::AudienceOnly` → `FollowersOnly`. Check uses the
  author's public follows (`list_public_follows`) rather than a separate
  audience table. `ModerationMode::AudienceOnly` similarly renamed.
- Follow/unfollow routines simplified: no audience downgrade logic;
  unfollow removes the social route entirely.

UI:
- CLI: `audience*` commands removed.
- Tauri: `AudienceDto`, `list_audience`, `list_audience_outbound`,
  `request_audience`, `approve_audience`, `remove_audience` commands
  removed from invoke_handler. Frontend: audience panel and audience/mutual
  badges removed; compose permission dropdown shows "Followers" instead of
  "Audience"; `loadAudience` is a no-op stub that hides any leftover DOM.

Tests: 111 / 111 core tests pass.

Breaking change: v0.6.2 nodes won't interoperate with v0.6.1 for delete
propagation, visibility updates, direct post push, post notifications, or
audience requests. Upgrade both ends.

crates/core/src/protocol.rs

@@ -32,10 +32,9 @@ pub enum MessageType {
     RefuseRedirect = 0x05,
     PullSyncRequest = 0x40,
     PullSyncResponse = 0x41,
-    PostNotification = 0x42,
-    PostPush = 0x43,
-    AudienceRequest = 0x44,
-    AudienceResponse = 0x45,
+    // 0x42 (PostNotification), 0x43 (PostPush), 0x44 (AudienceRequest),
+    // 0x45 (AudienceResponse) retired in v0.6.2: persona-signed direct pushes
+    // are gone. Public posts propagate via the CDN; encrypted posts via pull.
     ProfileUpdate = 0x50,
     DeleteRecord = 0x51,
     VisibilityUpdate = 0x52,
@@ -90,10 +89,6 @@ impl MessageType {
             0x05 => Some(Self::RefuseRedirect),
             0x40 => Some(Self::PullSyncRequest),
             0x41 => Some(Self::PullSyncResponse),
-            0x42 => Some(Self::PostNotification),
-            0x43 => Some(Self::PostPush),
-            0x44 => Some(Self::AudienceRequest),
-            0x45 => Some(Self::AudienceResponse),
             0x50 => Some(Self::ProfileUpdate),
             0x51 => Some(Self::DeleteRecord),
             0x52 => Some(Self::VisibilityUpdate),
@@ -241,32 +236,6 @@ pub struct VisibilityUpdatePayload {
     pub updates: Vec<VisibilityUpdate>,
 }
 
-/// Post notification: lightweight push when a new post is created
-#[derive(Debug, Serialize, Deserialize)]
-pub struct PostNotificationPayload {
-    pub post_id: PostId,
-    pub author: NodeId,
-}
-
-/// Audience request: ask a peer to join their audience
-#[derive(Debug, Serialize, Deserialize)]
-pub struct AudienceRequestPayload {
-    pub requester: NodeId,
-}
-
-/// Audience response: approve or deny an audience request
-#[derive(Debug, Serialize, Deserialize)]
-pub struct AudienceResponsePayload {
-    pub responder: NodeId,
-    pub approved: bool,
-}
-
-/// Post push: full post content pushed directly to a recipient
-#[derive(Debug, Serialize, Deserialize)]
-pub struct PostPushPayload {
-    pub post: SyncPost,
-}
-
 /// Address resolution request (bi-stream: ask reporter for a hop-2 peer's address)
 #[derive(Debug, Serialize, Deserialize)]
 pub struct AddressRequestPayload {
@@ -770,10 +739,6 @@ mod tests {
             MessageType::RefuseRedirect,
             MessageType::PullSyncRequest,
             MessageType::PullSyncResponse,
-            MessageType::PostNotification,
-            MessageType::PostPush,
-            MessageType::AudienceRequest,
-            MessageType::AudienceResponse,
             MessageType::ProfileUpdate,
             MessageType::DeleteRecord,
             MessageType::VisibilityUpdate,