fix: Rate-limited batches falsely trigger 'all processing layers exhausted'

[deucebucket]

Problem

When AI providers are rate-limited or circuit-broken, Layer 4 processing marks books as "All processing layers exhausted" even though the books are perfectly identifiable - they just couldn't be reached at that moment.

Merijeek reported (#150) a pile of "exhausted" books that were ALL previously identified successfully (McClellans, Alanson, Of Monsters and Mainframes, etc).

Root Cause

Two-part failure:

1. Double rate-limit check race condition

• worker.py:378-385 checks user rate limit, waits if needed ✓
• layer_ai_queue.py:100-103 checks AGAIN and returns (0, 0) immediately if not allowed
• Back in worker.py:394, processed == 0 increments empty_batch_count
• After 3 empty batches → everything marked as exhausted (worker.py:407-431)

2. Circuit-broken providers counted as "empty batch"

• If Gemini/OpenRouter are circuit-broken, call_ai() returns empty results
• process_queue() returns processed=0 even though items exist in the queue
• This counts toward the 3-strike exhaustion rule
• System doesn't distinguish "provider unavailable" from "genuinely unidentifiable"

Expected Behavior

• Rate-limited or circuit-broken batches should NOT count toward empty_batch_count
• If all providers are temporarily unavailable, system should WAIT for circuit breakers to clear, then retry
• Only count a batch as "empty" if providers were available and still couldn't identify
• Consider: requeue for retry later instead of permanent "exhausted" marking

Files Involved

• library_manager/worker.py:370-443 - Layer 4 loop with empty_batch_count logic
• library_manager/pipeline/layer_ai_queue.py:100-103 - Early return on rate limit
• library_manager/providers/rate_limiter.py - Circuit breaker state
• library_manager/providers/gemini.py:501-509 - Gemini 429 handling
• library_manager/providers/bookdb.py:153-159 - BookDB 429 handling

Reported By

@Merijeek in #150 (Feb 16) - books previously identified correctly now showing as exhausted

[deucebucket]

Fixed in PR #162 (merged to develop, beta.130). Rate-limited batches no longer count toward the 3-strike exhaustion rule. Circuit breaker awareness added to Layer 4.