refactor appview to use envvars. move distribution configurations to code

docs/BYOS.md

@@ -471,12 +471,30 @@ A company wants shared storage for their team:
 3. **In-memory cache** - Hold endpoint cache is in-memory (for production, use Redis)
 4. **Manual profile updates** - No UI for updating sailor profile (must use ATProto client)
 
+## Performance Optimization: S3 Presigned URLs
+
+**Status:** Planned implementation (see [PRESIGNED_URLS.md](./PRESIGNED_URLS.md))
+
+Currently, hold services act as proxies for blob data. With presigned URLs:
+
+- **Downloads:** Docker → S3 direct (via 307 redirect)
+- **Uploads:** Docker → AppView → S3 (via presigned URL)
+- **Hold service bandwidth:** Reduced by 99.98% (only orchestration)
+
+**Benefits:**
+- Hold services can run on minimal infrastructure ($5/month instances)
+- Direct S3 transfers at maximum speed
+- Scales to arbitrarily large images
+- Works with Storj, MinIO, Backblaze B2, Cloudflare R2
+
+See [PRESIGNED_URLS.md](./PRESIGNED_URLS.md) for complete technical details and implementation guide.
+
 ## Future Improvements
 
-1. **Automatic failover** - Multiple storage endpoints, fallback to default
-2. **Storage analytics** - Track usage per DID
-3. **Quota integration** - Optional quota tracking in storage service
-4. **Direct presigned URL support** - S3 native presigned URLs (bypass proxy)
+1. **S3 Presigned URLs** - Implement direct S3 URLs (see [PRESIGNED_URLS.md](./PRESIGNED_URLS.md))
+2. **Automatic failover** - Multiple storage endpoints, fallback to default
+3. **Storage analytics** - Track usage per DID
+4. **Quota integration** - Optional quota tracking in storage service
 5. **Profile management UI** - Web interface for users to manage their sailor profile
 6. **Distributed cache** - Redis/Memcached for hold endpoint cache in multi-instance deployments