v0.4.1: Security hardening, lock contention fixes, data cleanup

Security:
- Reaction signatures: ed25519 sign/verify (sign_reaction, verify_reaction_signature)
  Backward-compatible — unsigned reactions from old nodes still accepted
- Comment signature verification: verify_comment_signature now called on receipt
- Reaction removal authorization: only reactor or post author can remove
- BlobHeader author verification: lookup actual author from storage, don't trust payload

Lock contention (4 fixes):
- ManifestPush discovery: cm lock released before PostFetch I/O
- Pull request handler: load under lock, filter without lock, brief re-lock for is_deleted
- Pull sender: split into two brief locks (store posts, then batch upstream+sync)
- Engagement checker: batch all chunk results, single lock for writes

Data cleanup:
- Post deletion cleans post_downstream, post_upstream, seen_engagement tables
- Added TODO-hardening.md documenting remaining DOS/security/lock/data issues

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

crates/core/src/crypto.rs

@@ -549,6 +549,7 @@ pub fn random_slot_noise(size: usize) -> Vec<u8> {
 
 const REACTION_WRAP_CONTEXT: &str = "itsgoin/private-reaction/v1";
 const COMMENT_SIGN_CONTEXT: &str = "itsgoin/comment-sig/v1";
+const REACTION_SIGN_CONTEXT: &str = "itsgoin/reaction-sig/v1";
 
 /// Encrypt a private reaction payload (only the post author can decrypt).
 /// Uses X25519 DH between reactor and author, then ChaCha20-Poly1305.
@@ -645,6 +646,47 @@ pub fn verify_comment_signature(
     verifying_key.verify(digest.as_bytes(), &sig).is_ok()
 }
 
+/// Sign a reaction: ed25519 over BLAKE3(reactor || post_id || emoji || timestamp_ms).
+pub fn sign_reaction(
+    seed: &[u8; 32],
+    reactor: &NodeId,
+    post_id: &PostId,
+    emoji: &str,
+    timestamp_ms: u64,
+) -> Vec<u8> {
+    let signing_key = SigningKey::from_bytes(seed);
+    let mut hasher = blake3::Hasher::new_derive_key(REACTION_SIGN_CONTEXT);
+    hasher.update(reactor);
+    hasher.update(post_id);
+    hasher.update(emoji.as_bytes());
+    hasher.update(&timestamp_ms.to_le_bytes());
+    let digest = hasher.finalize();
+    signing_key.sign(digest.as_bytes()).to_bytes().to_vec()
+}
+
+/// Verify a reaction's ed25519 signature.
+pub fn verify_reaction_signature(
+    reactor: &NodeId,
+    post_id: &PostId,
+    emoji: &str,
+    timestamp_ms: u64,
+    signature: &[u8],
+) -> bool {
+    let Ok(verifying_key) = VerifyingKey::from_bytes(reactor) else {
+        return false;
+    };
+    let Ok(sig) = ed25519_dalek::Signature::from_slice(signature) else {
+        return false;
+    };
+    let mut hasher = blake3::Hasher::new_derive_key(REACTION_SIGN_CONTEXT);
+    hasher.update(reactor);
+    hasher.update(post_id);
+    hasher.update(emoji.as_bytes());
+    hasher.update(&timestamp_ms.to_le_bytes());
+    let digest = hasher.finalize();
+    verifying_key.verify(digest.as_bytes(), &sig).is_ok()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;