refactor: migrate sticker cache to PostgreSQL, remove versioning

- Replace file-based sticker cache (.dat + index.json) with PostgreSQL sticker_cache table
- Remove model_version column and all versioning logic (git branch detection, GitHub API, CACHE_MODEL_VERSION)
- Strip VISION_MODEL_VERSION, logCacheEvent calls, and version filtering from SQL queries
- Remove STICKER_CACHE_DIR and STICKER_CACHE_MAX_SIZE_MB config variables
- Delete dead migration add_model_version_to_cache.sql

textCacheStore.ts reduced 482→204 lines (-57%)
stickerCache.ts reduced 210→144 lines (-31%)
Total: 11 files changed, 233 insertions, 591 deletions

services/discord-gateway/src/modules/ai-moderation/responseLogger.ts

@@ -43,7 +43,6 @@ export interface CacheHitEvent {
   type: "hit" | "miss";
   cacheKey: string;
   source: "text" | "media" | "sticker";
-  modelVersion: string;
   timestamp: number;
 }
 
@@ -154,27 +153,22 @@ export function logCacheEvent(
   type: "hit" | "miss",
   cacheKey: string,
   source: "text" | "media" | "sticker",
-  modelVersion: string,
 ): void {
   const event: CacheHitEvent = {
     type,
     cacheKey,
     source,
-    modelVersion,
     timestamp: Date.now(),
   };
 
-  const level = type === "hit" ? "debug" : "debug";
-  logger.log(
-    { level },
+  logger.debug(
     {
       cache_type: type.toUpperCase(),
       source,
       key_length: cacheKey.length,
       key_preview: cacheKey.substring(0, 50),
-      model_version: modelVersion,
     },
-    `Cache ${type.toUpperCase()}: ${source} (version: ${modelVersion})`,
+    `Cache ${type.toUpperCase()}: ${source}`,
   );
 }