Add `prune` operation to v2 API standard operations

[fregataa]

Background

Backend.AI accumulates DB records of terminal-state entities over time (terminated sessions/kernels, delete-complete vfolders, etc.) along with their RBAC associations in association_scopes_entities. There is currently no first-class operation to clean these records in a controlled, audited way. This epic introduces a standard prune operation in v2 API to address this.

Goal

Add prune as the 7th standard operation in v2 API, alongside create, get, search, update, delete, and purge. prune is a bulk sweep that removes records matching a hardcoded terminal-state condition per entity, optionally bounded by a time argument.

prune is distinct from purge: purge hard-deletes a single specific instance by ID, while prune sweeps all qualifying records of an entity type.

Scope

Per-entity prune endpoints

POST /api/rest/v2/{entity}/prune

• Terminal-state condition is hardcoded per entity (caller cannot override via status filter).
• Optional until / before argument bounded by an entity-specific timestamp column (e.g., terminated_at for session/kernel, deleted_at for vfolder).
• Optional limit argument to cap rows processed per call.
• RBAC cascade option (default ON): when ON, matching rows in association_scopes_entities are deleted in the same transaction (association first, entity second).
• Response shape: {count: int, ids: [UUID, ...]}.

Standalone RBAC prune endpoint

POST /api/rest/v2/rbac/prune

• Removes orphan rows in association_scopes_entities whose (entity_type, entity_id) no longer exists in the target entity table.
• No filter args other than limit.
• Requires a registry mapping entity_type -> target table for polymorphic lookup.
• Acts as a safety net; under normal operation, per-entity prune (RBAC cascade ON by default) keeps orphans from accumulating.

Common requirements

• Authorization: superadmin only (admin_only).
• Scope: global (no domain/project scoping).
• Audit log: each prune call records caller, entity type, until argument, count, and id list.
• Concurrency: single transaction with DELETE ... RETURNING; PostgreSQL row locks naturally serialize concurrent callers.
• CLI: each prune endpoint exposed via ./bai v2 CLI.

Out of scope

• Background sweepers / cron-based automatic pruning (operator-triggered only).
• Retention policies (immediate prune; no "wait N days after termination" logic).
• Per-instance hard-delete via prune (use purge).
• Recovery / undo of pruned records.

Initial target entities

• Session / Kernel (terminal: terminated).
• vfolder (terminal: delete-complete).
• Additional entities to be enumerated as child stories.

Open items for child stories

• Per-entity terminal-state condition and timestamp column for until filter.
• entity_type -> table registry for RBAC orphan prune.
• CLI flag naming for the RBAC cascade option.

JIRA Issue: BA-5935