perf(desktop): move observer-feed archive and decrypt commands off main thread

[wpfleger96]

Opening the agent observer feed could beachball the app. In Tauri 2, a sync (pub fn) command body runs on the main thread — only async fn commands run on the runtime pool. Five commands on the observer-feed open path were sync, so panel open ran SQLite I/O and secp256k1 work on the macOS main thread:

Command	Main-thread work
decrypt_observer_event	Schnorr ID + signature verify, then NIP-44 decrypt — once per frame
read_archived_observer_events_for_channel	Opens the archive DB, runs the channel-index JOIN, returns up to 200 raw JSON blobs per page
read_unindexed_observer_rows	Opens the DB, returns all not-yet-indexed kind-24200 rows in one shot
index_observer_channel_id	Opens the DB, loops N upserts
delete_save_subscription	Opens the DB, one delete

Eager hydration loads up to 10 pages × 200 frames on panel open, so that's up to 10 main-thread DB reads plus up to 2,000 sequential verify+decrypt calls before any scrolling. The one-shot backfill makes it worse on the first open after history accumulates: one read of every unindexed row, a decrypt per row, then a batch upsert — all on the main thread, and all proportional to archive size.

The four archive commands now route their DB work through the existing run_archive_db_task helper (spawn_blocking + open_db), matching list_save_subscriptions, read_archived_events, and archive_events directly around them. decrypt_observer_event becomes async fn + tauri::async_runtime::spawn_blocking, with state.signing_keys() extracted before the spawn since State is not Send — the same pattern sign_event uses from #1222.

No frontend changes: invoke is already promise-based, so the TS wrappers in tauriArchive.ts and tauriObserver.ts are unchanged.

This removes the freeze, not the work. Eager hydration still takes the same wall time — the feed shows a loading state instead of blocking the UI. Batching the per-frame decrypt IPC (2,000 round-trips into one command) would cut the latency itself; that's deliberately out of scope here.