fix: correct type mismatches in DLQ process_failed_jobs (#3)

Fixes two type mismatches when reading from graphile_worker's jobs
table:

1. **priority**: Changed from `i32` to `i16` to match graphile_worker's
`smallint` type
2. **last_error**: Changed from `Option<serde_json::Value>` to
`Option<String>` to match graphile_worker's `TEXT` type, with conversion
to JSONB for DLQ insert

## Problem

These mismatches caused the worker to crash with ColumnDecode errors
when attempting to move failed jobs to the DLQ:
- priority: `Rust type 'i32' is not compatible with SQL type 'INT2'`
- last_error: `Rust type 'Option<serde_json::Value>' is not compatible
with SQL type 'TEXT'`

## Root Cause

The graphile_worker library uses `smallint` (INT2/i16) for priority and
`TEXT` for last_error, while the backfill_dlq table expects `INTEGER`
and `JSONB` respectively.

## Solution

This fix reads the correct types from the source table and converts
`last_error` from TEXT to JSONB before inserting into DLQ.

## Additional Fixes

To make CI pass, this PR also includes fixes for pre-existing issues:
- Fixed clippy warning: derive `Default` for `Queue` enum instead of
manual implementation
- Fixed doctest failures: added missing `Serialize`/`Deserialize`
derives and explicit type annotations

## Testing

All DLQ tests pass (18/18):
- `test_dlq_process_failed_jobs_empty`
- `test_dlq_process_failed_jobs_with_mock_data`
- And 16 other DLQ-related tests

Author: rrogers-machinify

src/client/dlq.rs

@@ -558,14 +558,17 @@ impl BackfillClient {
             let payload = serde_json::json!({});
             let queue_name: Option<String> = job_row.get("queue_name");
             let queue_name = queue_name.unwrap_or_else(|| "default".to_string());
-            let priority: i32 = job_row.get("priority");
+            let priority: i16 = job_row.get("priority");
             let job_key: Option<String> = job_row.get("job_key");
             let max_attempts: i16 = job_row.get("max_attempts");
             let attempts: i16 = job_row.get("attempts");
-            let last_error: Option<serde_json::Value> = job_row.get("last_error");
+            let last_error: Option<String> = job_row.get("last_error");
             let created_at: chrono::DateTime<chrono::Utc> = job_row.get("created_at");
             let run_at: chrono::DateTime<chrono::Utc> = job_row.get("run_at");
 
+            // Convert last_error from TEXT to JSONB for DLQ table
+            let last_error_json = last_error.map(serde_json::Value::String);
+
             // Move to DLQ
             let insert_dlq_query = format!(
                 r#"
@@ -590,7 +593,7 @@ impl BackfillClient {
                 .bind(max_attempts as i32)
                 .bind(failure_reason)
                 .bind(attempts as i32)
-                .bind(&last_error)
+                .bind(&last_error_json)
                 .bind(created_at)
                 .bind(run_at)
                 .execute(&self.pool)

src/lib.rs

@@ -85,11 +85,12 @@ pub use retries::*;
 pub use worker::*;
 
 /// Named queues for organizing different types of work.
-#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, PartialEq, Eq, Hash, Default)]
 pub enum Queue {
     /// Fast queue for high-priority, low-latency jobs
     Fast,
     /// Bulk queue for background processing
+    #[default]
     Bulk,
     /// Dead letter queue for failed jobs
     DeadLetter,
@@ -108,12 +109,6 @@ impl Queue {
     }
 }
 
-impl Default for Queue {
-    fn default() -> Self {
-        Self::Bulk
-    }
-}
-
 /// Configuration for job scheduling and execution.
 #[derive(Debug, Clone)]
 pub struct JobSpec {