feat(queue): Add generic dependent fetch queue with exponential backoff

New module src/core/dependent_queue.rs provides job queue operations
against the pending_dependent_fetches table. Designed for second-pass
fetches that depend on primary entity ingestion (resource events,
MR close references, MR file diffs).

Queue operations:
- enqueue_job: Idempotent INSERT OR IGNORE keyed on the UNIQUE
  (project_id, entity_type, entity_iid, job_type) constraint.
  Returns bool indicating whether the row was actually inserted.

- claim_jobs: Two-phase claim — SELECT available jobs (unlocked,
  past retry window) then UPDATE locked_at in batch. Orders by
  enqueued_at ASC for FIFO processing within a job type.

- complete_job: DELETE the row on successful processing.

- fail_job: Increments attempts, calculates exponential backoff
  (30s * 2^(attempts-1), capped at 480s), sets next_retry_at,
  clears locked_at, and records the error message. Reads current
  attempts via query with unwrap_or(0) fallback for robustness.

- reclaim_stale_locks: Clears locked_at on jobs locked longer than
  a configurable threshold, recovering from worker crashes.

- count_pending_jobs: GROUP BY job_type aggregation for progress
  reporting and stats display.

Registers both events_db and dependent_queue in src/core/mod.rs.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/core/mod.rs

@@ -3,7 +3,9 @@
 pub mod backoff;
 pub mod config;
 pub mod db;
+pub mod dependent_queue;
 pub mod error;
+pub mod events_db;
 pub mod lock;
 pub mod paths;
 pub mod payloads;