fix: staging CI review issues (batch 1) (#883)

* fix: address staging-ci-review issues (batch 1)

- #811: Fix unreachable error handling in worker — restructure .await?
  to explicit match on nested Result so token budget errors are properly
  logged and marked as failed
- #813: Combine metadata + token budget into single update_context()
  call to prevent concurrent worker observing partial state
- #814: Persist max_tokens and total_tokens_used to both PostgreSQL and
  libSQL backends — add V12 migration, update save_job/get_job
- #815: Cap user-supplied max_tokens at configured max_tokens_per_job
  to prevent budget bypass via metadata injection
- #869: Release locks before async I/O in webhook handler (http.rs) and
  SIGHUP handler (main.rs) to prevent blocking concurrent requests

Fixes: #811, #813, #814, #815, #869

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* fix: address PR #883 review feedback

- Fix min(user_val, 0) bug: guard for unlimited config (max_tokens_per_job == 0)
- Remove duplicate columns from libSQL base SCHEMA (v12 migration is sole source)
- Use get_i64() helper for consistency in libsql/jobs.rs
- Add regression tests for scheduler token budget capping

Co-Authored-By: Claude Opus 4.6 <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 <[email protected]>

src/channels/http.rs

@@ -395,9 +395,14 @@ async fn process_message(
         None
     };
 
-    // Send message to the channel
-    let tx_guard = state.tx.read().await;
-    if let Some(tx) = tx_guard.as_ref() {
+    // Clone sender while holding read lock, then release lock before async send.
+    // This prevents blocking other webhook handlers during the async I/O.
+    let tx = {
+        let guard = state.tx.read().await;
+        guard.as_ref().cloned()
+    };
+
+    if let Some(tx) = tx {
         if tx.send(msg).await.is_err() {
             return (
                 StatusCode::INTERNAL_SERVER_ERROR,
@@ -418,7 +423,6 @@ async fn process_message(
             }),
         );
     }
-    drop(tx_guard);
 
     // Wait for response if requested
     let response = if let Some(rx) = response_rx {