v0.4.0: Protocol v4 — header-driven sync, tiered engagement, multi-upstream

Protocol v4 sync overhaul:
- Slim PullSyncRequest: per-author timestamps (since_ms) replace full post ID lists
  Request size O(follows) instead of O(posts). Backward-compatible via serde default.
- Tiered pull frequency: 60s ticks, only syncs stale authors (4hr default)
  Full pull only on first tick (bootstrap). Most ticks skip — no stale authors.
- Tiered engagement checks: frequency scales with content age
  5min (<72h), 1hr (3-14d), 4hr (14-30d), 24hr (>30d)
  Single SQL query filters posts due for check.
- Header-driven post discovery: ManifestPush triggers PostFetch for missing
  followed-author posts (capped 10 per manifest). CDN tree = notification system.
- Multi-upstream (3 max): composite PK, priority ordering, engagement diffs
  sent to all upstreams, promote/remove on failure.

DB schema:
- follows.last_sync_ms — Self Last Encounter per author
- posts.last_engagement_ms — last reaction/comment timestamp
- posts.last_check_ms — last engagement check timestamp
- post_upstream: single-row → 3-row with priority column

Lock contention fixes:
- get_blob_for_post: 3 locks → 1
- prefetch_blobs_from_peer: lock-free blob checks
- fetch_engagement_from_peer: explicit lock release before I/O
- serve_post: 4 locks → 2 (eliminated redundant queries)
- run_replication_check: 2 locks → 1
- Badge cycle: N+2 IPC calls → 1 (get_badge_counts)

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

crates/tauri-app/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "itsgoin-desktop"
-version = "0.3.6"
+version = "0.4.0"
 edition = "2021"
 
 [lib]