add cache only mode to hosting service

hosting-service/src/index.ts

···
 import { logger } from './lib/observability';
 import { mkdirSync, existsSync } from 'fs';
 import { backfillCache } from './lib/backfill';
-import { startDomainCacheCleanup, stopDomainCacheCleanup } from './lib/db';
+import { startDomainCacheCleanup, stopDomainCacheCleanup, setCacheOnlyMode } from './lib/db';
 
 const PORT = process.env.PORT ? parseInt(process.env.PORT) : 3001;
 const CACHE_DIR = process.env.CACHE_DIR || './cache/sites';
···
 const args = process.argv.slice(2);
 const hasBackfillFlag = args.includes('--backfill');
 const backfillOnStartup = hasBackfillFlag || process.env.BACKFILL_ON_STARTUP === 'true';
+
+// Cache-only mode: service will only cache files locally, no DB writes
+const hasCacheOnlyFlag = args.includes('--cache-only');
+export const CACHE_ONLY_MODE = hasCacheOnlyFlag || process.env.CACHE_ONLY_MODE === 'true';
+
+// Configure cache-only mode in database module
+if (CACHE_ONLY_MODE) {
+  setCacheOnlyMode(true);
+}
 
 // Ensure cache directory exists
 if (!existsSync(CACHE_DIR)) {
···
 Health:       http://localhost:${PORT}/health
 Cache:        ${CACHE_DIR}
 Firehose:     Connected to Firehose
+Cache-Only:   ${CACHE_ONLY_MODE ? 'ENABLED (no DB writes)' : 'DISABLED'}
 `);
 
 // Graceful shutdown

hosting-service/src/lib/db.ts

···
 import postgres from 'postgres';
 import { createHash } from 'crypto';
 
+// Global cache-only mode flag (set by index.ts)
+let cacheOnlyMode = false;
+
+export function setCacheOnlyMode(enabled: boolean) {
+  cacheOnlyMode = enabled;
+  if (enabled) {
+    console.log('[DB] Cache-only mode enabled - database writes will be skipped');
+  }
+}
+
 const sql = postgres(
   process.env.DATABASE_URL || 'postgres://postgres:postgres@localhost:5432/wisp',
   {
···
 }
 
 export async function upsertSite(did: string, rkey: string, displayName?: string) {
+  // Skip database writes in cache-only mode
+  if (cacheOnlyMode) {
+    console.log('[DB] Skipping upsertSite (cache-only mode)', { did, rkey });
+    return;
+  }
+
   try {
     // Only set display_name if provided (not undefined/null/empty)
     const cleanDisplayName = displayName && displayName.trim() ? displayName.trim() : null;

hosting-service/src/lib/firehose.ts

···
 		)
 
 		// Acquire distributed lock only for database write to prevent duplicate writes
+		// Note: upsertSite will check cache-only mode internally and skip if needed
 		const lockKey = `db:upsert:${did}:${site}`
 		const lockAcquired = await tryAcquireLock(lockKey)
 
···
 
 		try {
 			// Upsert site to database (only one instance does this)
+			// In cache-only mode, this will be a no-op
 			await upsertSite(did, site, fsRecord.site)
 			this.log(
 				'Successfully processed create/update (cached + DB updated)',