fix: correct type mismatches in DLQ process_failed_jobs

[rrogers-machinify]

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