Fix batch assembly timeout, progress tracking, and error reporting

[jjroelofs]

Summary

Fixes #18. Three bugs in the centralized batch processing infrastructure:

• Assembly timeout: getFullyAnalyzedEntityIds() rendered every entity in the bundle via hasResults() just to build an exclusion list, causing 504s even with limit=1. Replaced with getAnalyzedEntityIds() DB queries that never load or render entities.
• Infinite loop: $context['finished'] was calculated as a fraction of total entities across all operations, but each chunk is a separate batch operation. Drupal re-invoked the same operation endlessly. Now sets finished = 1 per chunk.
• Silent failures: batchFinished() only reported processed count. Now reports failed count and individual error messages.

Changes

• Add getAnalyzedEntityIds() to BatchableAnalyzerInterface with default [] in AnalyzePluginBase
• Rewrite getFullyAnalyzedEntityIds() to intersect per-analyzer DB results
• Fix $context['finished'] = 1 at end of each chunk operation
• Report failures and errors prominently in batchFinished()

Test plan

• Run batch with limit < total entities, verify batch completes and advances through all chunks
• Run batch on a bundle with 1000+ entities, verify no 504 during form submission
• Trigger an entity rendering error, verify it appears as a named error in the batch results
• Run batch with force refresh off on already-analyzed content, verify entities are correctly skipped

[jjroelofs]

Splitting into separate PRs, one per fix.