fix(pipeline): fix priority

[petkostas]

Summary

After upgrading from v1.0.2-beta8 to v1.0.3-beta7, DevLake instances crash on startup with the following error:

│ [GIN-debug] Listening and serving HTTP on :8080                                                      │
│ panic: sql: Scan error on column index 0, name "priority": converting NULL to int is unsupported (50 │
│ Wraps: (2) sql: Scan error on column index 0, name "priority"                                        │
│ Wraps: (3) converting NULL to int is unsupported                                                     │
│ Error types: (1) *hintdetail.withDetail (2) *fmt.wrapError (3) *errors.errorString                   │
│                                                                                                      │
│ goroutine 116 [running]:                                                                             │
│ github.com/apache/incubator-devlake/server/services.dequeuePipeline({0x2c8cb78, 0x0, 0x0})           │
│     /app/server/services/pipeline.go:270 +0x10d3                                                     │
│ github.com/apache/incubator-devlake/server/services.RunPipelineInQueue(0x1)                          │
│     /app/server/services/pipeline.go:335 +0x42c                                                      │
│ created by github.com/apache/incubator-devlake/server/services.pipelineServiceInit                   │
│     /app/server/services/pipeline.go:112 +0x23d

This occurs in dequeuePipeline() when attempting to query the priority column.
The migration script 20250813_add_pipeline_priority.go adds the priority column to existing tables but doesn't set default values for existing rows, leaving them as NULL.
When the pipeline service queries these NULL values into Go's int type (which is not nullable), it causes a panic.

Does this close any open issues?

Closes #8621

[klesh]

Thanks for fixing the issue.