v0.3.6: Active CDN replication, device roles, budgets, tombstones, engagement fix, DOS hardening

Active CDN replication:
- All devices proactively replicate recent posts (<72h, <2 replicas) to peers
- Target priority: desktops (300) > anchors (200) > phones (100) + cache_pressure
- ReplicationRequest/Response (0xE1/0xE2) wire messages
- 10-min cycle, 2-min initial delay, cap 20 posts per request
- Graceful with small networks (1 peer = 1 replica, 0 peers = silent skip)

Device roles & budgets:
- Intermittent (phone), Available (desktop), Persistent (anchor)
- Advertised in InitialExchange, stored per-peer
- Replication budget: phones 100MB/hr, desktops/anchors 200MB/hr
- Delivery budget: phones 1GB/hr, desktops 2GB/hr, anchors 1GB/hr
- Hourly auto-reset, enforcement on blob serving

Cache management:
- 1GB default cache limit, configurable in settings UI
- Eviction cycle activated (was implemented but never started)
- Share-link priority boost (+100 for 3+ downstream)
- Cache pressure score (0-255) for replication targeting

Engagement distribution fix:
- BlobHeader JSON rebuilt after BlobHeaderDiff ops
- Previously reactions/comments stored in tables but header stayed stale

Tombstone system:
- deleted_at column on reactions and comments
- Tombstones propagate through pull sync (additive merge respects timestamps)
- UI queries filter WHERE deleted_at IS NULL

Persistent notifications:
- seen_engagement and seen_messages tables replace in-memory Sets
- Only notify on genuinely unseen content, survives restarts

DOS hardening:
- BlobHeaderDiff fan-out: single batched task, max 10 concurrent via JoinSet
- Blob prefetch: cap 20 per cycle, newest first
- PostDownstreamRegister: cap 50 per sync
- Delivery budget enforcement on BlobRequest handler
- Pull preference: non-anchors first to preserve anchor delivery budget

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/core/src/protocol.rs

@@ -71,6 +71,8 @@ pub enum MessageType {
     TcpPunchRequest = 0xD6,
     TcpPunchResult = 0xD7,
     MeshKeepalive = 0xE0,
+    ReplicationRequest = 0xE1,
+    ReplicationResponse = 0xE2,
 }
 
 impl MessageType {
@@ -126,6 +128,8 @@ impl MessageType {
             0xD6 => Some(Self::TcpPunchRequest),
             0xD7 => Some(Self::TcpPunchResult),
             0xE0 => Some(Self::MeshKeepalive),
+            0xE1 => Some(Self::ReplicationRequest),
+            0xE2 => Some(Self::ReplicationResponse),
             _ => None,
         }
     }
@@ -174,6 +178,12 @@ pub struct InitialExchangePayload {
     /// External HTTP address if known (e.g. "1.2.3.4:4433")
     #[serde(default)]
     pub http_addr: Option<String>,
+    /// CDN replication device role: "intermittent", "available", "persistent"
+    #[serde(default)]
+    pub device_role: Option<String>,
+    /// CDN cache pressure: 0-255 availability score (255 = lots of capacity)
+    #[serde(default)]
+    pub cache_pressure: Option<u8>,
 }
 
 /// Incremental N1/N2 changes
@@ -622,6 +632,26 @@ pub struct PostFetchResponsePayload {
     pub post: Option<SyncPost>,
 }
 
+// --- Active CDN Replication payloads ---
+
+/// Request a peer to replicate (cache) specific posts and their blobs (bi-stream)
+#[derive(Debug, Serialize, Deserialize)]
+pub struct ReplicationRequestPayload {
+    /// Posts to replicate (with their blobs)
+    pub post_ids: Vec<PostId>,
+    /// 0-255 urgency (higher = more important to cache)
+    pub priority: u8,
+}
+
+/// Response to a replication request (bi-stream)
+#[derive(Debug, Serialize, Deserialize)]
+pub struct ReplicationResponsePayload {
+    /// Posts the peer agreed to hold
+    pub accepted: Vec<PostId>,
+    /// Posts the peer declined (over budget or no space)
+    pub rejected: Vec<PostId>,
+}
+
 /// Request a TCP hole punch toward a browser IP (bi-stream).
 /// Sent by the anchor to a node that holds a post, so the node's NAT
 /// opens a pinhole allowing the browser to connect directly via HTTP.
@@ -766,6 +796,11 @@ mod tests {
             MessageType::PostDownstreamRegister,
             MessageType::PostFetchRequest,
             MessageType::PostFetchResponse,
+            MessageType::TcpPunchRequest,
+            MessageType::TcpPunchResult,
+            MessageType::MeshKeepalive,
+            MessageType::ReplicationRequest,
+            MessageType::ReplicationResponse,
         ];
         for mt in types {
             let byte = mt.as_byte();