perf(actions): match runner tasks in SQL via normalized job-label table

[bircni]

Split out of #38150, which is being broken into smaller, independently reviewable PRs. This one contains the database part: the schema and query changes that make runner task assignment scale with the waiting backlog. It is independent of the no-DB split (#38281) and can land in either order.

Runner task assignment previously loaded every waiting job and filtered labels in Go. This replaces that with an indexed SQL query so a poll stays ~O(1 row) regardless of backlog size:

• Normalized action_run_job_label table — one row per RunsOn label, kept in sync on job insert/delete, so label matching happens in SQL instead of in memory.
• Composite (status, updated) index on action_run_job so "oldest waiting job" is an index seek instead of a sort of the whole waiting backlog.
• Rewritten CreateTaskForRunner around the SQL match; unpreparable jobs are failed so they leave the queue, and run/attempt status is re-aggregated on claim so run-level concurrency still sees the occupying run.
• Migration v343 creates the table, adds the index, and backfills labels for assignable (waiting/blocked) jobs.

[Copilot]

Pull request overview

This PR introduces database-backed runner label matching and queue-pick optimizations for Gitea Actions task assignment. It normalizes runs-on labels into a dedicated table and rewrites task picking to select the oldest matchable waiting job in SQL, avoiding O(backlog) in-memory filtering and preventing head-of-line stalls from unpreparable jobs.

Changes:

• Add normalized action_run_job_label table plus migration/backfill (v343) and add a composite (status, updated) index to speed up “oldest waiting job” selection.
• Route ActionRunJob creation through a single insert path that also persists label rows, and add label cleanup on run/repo deletion paths to avoid orphaned label rows.
• Rewrite CreateTaskForRunner into a transactional SQL-based picker with bounded skip/fail handling for unpreparable jobs, plus new/updated tests covering matching and failure behavior.

Reviewed changes

Copilot reviewed 15 out of 16 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
services/repository/delete.go	Deletes job-label rows before repo-scoped job deletion to avoid orphan labels.
services/actions/run.go	Inserts jobs via InsertActionRunJob to keep label projection in sync.
services/actions/reusable_workflow.go	Uses InsertActionRunJob for reusable-workflow child job inserts.
services/actions/rerun.go	Uses InsertActionRunJob when cloning jobs for reruns to keep labels synced.
services/actions/cleanup.go	Deletes job-label rows before deleting run jobs during run cleanup.
models/migrations/v1_28/v343.go	Migration creating label table, adding composite index, and backfilling labels for waiting/blocked jobs.
models/migrations/v1_28/v343_test.go	Migration test validating backfill behavior (including dedup).
models/migrations/v1_28/main_test.go	TestMain bootstrap for v1_28 migration tests.
models/migrations/migrations.go	Registers migration 343.
models/fixtures/action_run_job_label.yml	Fixture file for the new label table (empty baseline).
models/actions/task.go	Removes the old in-memory scanning picker implementation (moved to task_pick.go).
models/actions/task_test.go	Adds tests validating SQL-based picking semantics and unpreparable-job handling.
models/actions/task_pick.go	New transactional SQL-based CreateTaskForRunner and helpers.
models/actions/run_job.go	Adds composite (status, updated) index tags used by the picker.
models/actions/run_job_label.go	New model + insert/delete helpers for normalized runs-on labels and SQL match condition builder.
models/actions/run_job_label_test.go	Tests for label matching contract and SQL condition equivalence.

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