feat(BA-5936): add Pruner repository abstraction

[fregataa]

Summary

• Introduce PrunerSpec[TRow] abstraction parallel to existing Creator/Updater/Purger patterns. Subclasses declaratively encode the entity's prune contract via row_class(), returning_id(), prune_condition(), and optional entity_type() classmethods; per-call params (conditions, cascade, limit, cascade_rbac) live on the instance.
• CascadeChild abstraction handles FK-dependent child tables (e.g., kernels of pruned sessions); entity_type() opts into RBAC association cleanup baked into execute_pruner for cross-cutting association_scopes_entities semantics.
• execute_pruner runs SELECT pk FOR UPDATE LIMIT once, materializes IDs, then issues cascade DELETEs and the parent DELETE...RETURNING from the same locked set — race-safe and avoids re-evaluating the parent SELECT per cascade. Hard-capped at DEFAULT_PRUNE_LIMIT = 100_000 to bound lock set, memory, and transaction duration.

Test plan

• pants test tests/unit/manager/repositories/base/test_pruner.py — 11 tests covering basic prune, runtime conditions, limit, FK cascade, RBAC cleanup with entity_type filtering, and the default no-op (entity_type=None) path.
• pants fmt, pants fix, pants lint, pants check all pass on changed files.
• CI: full type check + test suite.

Notes

This adds the abstraction only. Concrete per-entity Pruner specs (session, vfolder, etc.) and the v2 REST prune endpoint follow as separate PRs under epic BA-5935.

Resolves BA-5936

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a new repository-layer “Pruner” abstraction for race-safe bulk deletions, aligned with existing Creator/Updater/Purger patterns, plus integration tests exercising cascading deletes and optional RBAC association cleanup.

Changes:

• Introduce PrunerSpec, CascadeChild, PrunerResult, and execute_pruner() to perform SELECT ... FOR UPDATE-based pruning with optional cascades.
• Add RBAC association cleanup support via PrunerSpec.entity_type() + cascade_rbac.
• Add real-DB integration tests covering basic pruning, runtime conditions, limit behavior, FK cascades, and RBAC cleanup.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
src/ai/backend/manager/repositories/base/pruner.py	New pruner abstraction + executor implementing lock-then-delete flow with cascade support.
src/ai/backend/manager/repositories/base/__init__.py	Re-export new pruner APIs from the base repository package.
tests/unit/manager/repositories/base/test_pruner.py	Integration tests validating pruning, cascade deletes, and RBAC cleanup behavior.

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