feat: add response logger for moderation/vision audit trail + cache model versioning

- Add responseLogger.ts with comprehensive logging: vision analysis, moderation
  analysis batches, cache hit/miss events, errors, retries, false positives,
  model version changes, cache invalidation
- Integrate responseLogger calls into llmModerationClient.ts (cache events,
  moderation results, errors, retries)
- Add model_version column to text_analysis_cache schema with composite
  index for efficient filtering
- Add migration SQL for model_version column
- Document CACHE_MODEL_VERSION in .env.example

services/discord-gateway/src/shared/database/migrations/add_model_version_to_cache.sql

@@ -0,0 +1,24 @@
+-- Migration: Add model_version column to text_analysis_cache table
+-- Purpose: Track which vision/LLM model version produced each cache entry
+-- Reason: Invalidate old cache entries when model prompts change (e.g., terminal screenshot false positive fix)
+-- Date: 2026-06-02
+
+BEGIN;
+
+-- Add model_version column with default value
+ALTER TABLE text_analysis_cache
+ADD COLUMN model_version VARCHAR(50) NOT NULL DEFAULT 'v1';
+
+-- Create index for efficient filtering by model version
+CREATE INDEX idx_text_analysis_cache_model_version
+ON text_analysis_cache(model_version);
+
+-- Create composite index for source + model_version queries (common pattern)
+CREATE INDEX idx_text_analysis_cache_source_model_version
+ON text_analysis_cache(source, model_version);
+
+-- Optional: Clean up old vision_llm entries that may have stale/incorrect analysis
+-- Uncomment to remove all old vision analysis cache on deployment:
+-- DELETE FROM text_analysis_cache WHERE source = 'vision_llm' AND model_version = 'v1';
+
+COMMIT;