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>

app/frontend/nginx.conf

@@ -21,6 +21,12 @@ http {
     keepalive_timeout 65;
     types_hash_max_size 2048;
 
+    # WebSocket upgrade support
+    map $http_upgrade $connection_upgrade {
+        default upgrade;
+        ''      close;
+    }
+
     # Gzip compression
     gzip on;
     gzip_vary on;
@@ -51,6 +57,22 @@ http {
         add_header Referrer-Policy "no-referrer-when-downgrade" always;
         add_header Content-Security-Policy "default-src 'self' http: https: data: blob: 'unsafe-inline'" always;
 
+        # Same-origin API proxy - keeps the published image host-agnostic
+        location /api/ {
+            resolver 127.0.0.11 valid=10s;
+            set $backend_upstream http://backend:8080;
+            proxy_pass $backend_upstream;
+            proxy_http_version 1.1;
+            proxy_set_header Host $http_host;
+            proxy_set_header X-Real-IP $remote_addr;
+            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+            proxy_set_header X-Forwarded-Proto $scheme;
+            proxy_set_header Upgrade $http_upgrade;
+            proxy_set_header Connection $connection_upgrade;
+            proxy_read_timeout 3600s;
+            proxy_send_timeout 3600s;
+        }
+
         # Handle React Router
         location / {
             try_files $uri $uri/ /index.html;