feat: swap Redis for Dragonfly, fix remote installs, self-hosted email-only auth

- Replace redis:7-alpine with Dragonfly (Redis-protocol compatible; go-redis
  client unchanged) in root, infra, and backend-dev compose files. Bounded
  defaults --proactor_threads/--maxmemory so it boots on small VPSs.
- Frontend now defaults to same-origin API calls; nginx proxies /api/* to the
  backend (WebSocket-aware, docker DNS resolver for upstream re-resolution).
  Removes the baked localhost:8082 that broke installs reached by IP/domain.
- /auth/bootstrap returns configured OAuth providers; the UI hides
  GitHub/Google buttons unless real credentials exist. Self-hosted stays
  email/password only - OAuth reserved for a future managed offering.
- Sidecar ignores PLACEHOLDER_* OAuth credentials and trusts same-host
  origins dynamically (Origin host == request Host) so remote installs pass
  origin checks without config; cross-origin requests still rejected.
- install.sh writes ENVIRONMENT=production; COOKIE_SECURE=false in
  production is now a startup warning instead of fatal (plain-HTTP installs).
- Bootstrap failure surfaces an error notice instead of silently rendering
  the sign-in page.

Authored By: TDvorak <[email protected]>

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

docs/guides/DOCKER_SETUP.md

@@ -68,7 +68,7 @@ Internet → Traefik (Port 80/443)
     ├── Frontend (React/Nginx)
     ├── Backend (Go API)
     ├── PostgreSQL (Database)
-    └── Redis (Cache)
+    └── Dragonfly (Cache)
 ```
 
 ## Configuration Files
@@ -138,8 +138,8 @@ docker-compose up -d --force-recreate
 # Backup PostgreSQL
 docker-compose exec postgres pg_dump -U containr_user containr > backup.sql
 
-# Backup Redis
-docker-compose exec redis redis-cli --rdb /data/dump.rdb
+# Backup Dragonfly (snapshot)
+docker-compose exec dragonfly redis-cli -a "$REDIS_PASSWORD" save
 ```
 
 ### SSL Certificates
@@ -211,7 +211,7 @@ docker-compose up -d
 ### Performance Tuning
 
 1. **Nginx Caching** - Already configured in `nginx.conf`
-2. **Redis Caching** - Configure in your application
+2. **Dragonfly Caching** - Configure in your application
 3. **Database Pooling** - Adjust connection limits in Go app
 
 ## Production Tips

docs/guides/README_BACKEND.md

@@ -5,7 +5,7 @@
 ### Prerequisites
 - Go 1.21+
 - PostgreSQL 12+
-- Redis (optional)
+- Dragonfly or Redis-compatible cache (optional)
 
 ### Environment Variables