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>

README.md

@@ -46,7 +46,7 @@ click away.
 - **Managed databases** — one-click provisioning with backup/restore
 - **Metrics** — per-service Docker stats plus host CPU/memory/disk telemetry
 - **Templates, cron, previews** — service catalog, scheduled jobs, preview deploys
-- **Auth & security** — Better Auth sessions (email + OAuth), audit logs, vuln scans
+- **Auth & security** — Better Auth sessions (email/password self-hosted), audit logs, vuln scans
 - **Networking** — Traefik reverse proxy, optional Cloudflare Tunnel
 
 ## Quick Start
@@ -60,6 +60,8 @@ curl -fsSL https://raw.githubusercontent.com/Dvorinka/Containr/main/install.sh |
 Installs into `./containr`, generates secrets, pulls the published `:latest`
 images, and starts the stack — UI at `http://localhost:3000`, API at
 `http://localhost:8082`. Non-interactive; re-runs never overwrite `.env`.
+On first visit you create the owner account (email/password); registration
+then closes automatically.
 
 Override with env vars:
 
@@ -80,7 +82,7 @@ see [docs/guides/](docs/guides/) and `infra/docker-compose.yml`.
 
 ```
 app/frontend/   React 19 + Vite + TypeScript + Tailwind + React Flow
-app/backend/    Go API (Gin) · PostgreSQL · Redis · Docker SDK · Better Auth sidecar
+app/backend/    Go API (Gin) · PostgreSQL · Dragonfly · Docker SDK · Better Auth sidecar
 infra/          Self-hosted compose: Traefik + Cloudflare Tunnel
 docs/           Guides, OpenAPI spec, design documents
 ```