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>

.env.example

@@ -1,4 +1,5 @@
-# Local development env file. Use .env.prod for production.
+# Local development env file. install.sh writes ENVIRONMENT=production for
+# self-hosted installs - keep development here only for the checkout workflow.
 
 # Domain Configuration
 DOMAIN=localhost
@@ -28,9 +29,12 @@ AUTO_MIGRATE=true
 MIGRATION_LOCK_TIMEOUT=2m
 SEED_DATA_ON_START=false
 
-# Redis Configuration
+# Dragonfly Configuration (Redis-compatible cache - REDIS_* names kept for compatibility)
 REDIS_PASSWORD=your_secure_redis_password
 REDIS_URL=redis://:your_secure_redis_password@localhost:6379/0
+# DRAGONFLY_VERSION=latest
+# DRAGONFLY_THREADS=1
+# DRAGONFLY_MAXMEMORY=256mb
 
 # Application Configuration
 # In production this must be a strong value with at least 32 characters.
@@ -40,8 +44,11 @@ CONTAINR_AGENT_AUTH_TOKEN=replace_with_strong_agent_secret
 # Optional rotation list (comma-separated). When set, this takes precedence.
 # CONTAINR_AGENT_AUTH_TOKENS=current_secret,next_secret
 CORS_ORIGINS=http://localhost:3000,http://localhost:8080
-VITE_API_URL=http://localhost:8082
-VITE_AUTH_URL=http://localhost:8082/api/auth
+# Optional build-time API URL overrides. Leave empty for the default
+# same-origin setup - the frontend nginx proxies /api to the backend, which
+# works from any host or IP without rebuilding the image.
+# VITE_API_URL=https://api.example.com
+# VITE_AUTH_URL=https://api.example.com/api/auth
 AUTH_PORT=3001
 # In production this must be true.
 COOKIE_SECURE=false
@@ -54,6 +61,8 @@ BETTER_AUTH_AUTO_MIGRATE=true
 BETTER_AUTH_INTERNAL_TOKEN=PLACEHOLDER_INTERNAL_AUTH_TOKEN
 BETTER_AUTH_INTERNAL_URL=http://127.0.0.1:3001/internal/session
 BETTER_AUTH_PROXY_URL=http://127.0.0.1:3001
+# Extra trusted origins (comma-separated). Same-origin traffic is trusted
+# automatically, so this only needs entries for cross-origin callers.
 BETTER_AUTH_TRUSTED_ORIGINS=http://localhost:3000,http://localhost:8082
 # Optional explicit auth DB settings (recommended when password contains URL special chars)
 DB_HOST=postgres
@@ -64,7 +73,9 @@ DB_PASSWORD=your_secure_postgres_password
 FRONTEND_URL=http://localhost:3000
 BACKEND_URL=http://localhost:8082
 
-# OAuth (user auth)
+# OAuth (user auth) - GitHub/Google sign-in is reserved for the managed
+# offering. Self-hosted installs use email/password only; PLACEHOLDER_*
+# values are treated as unset and the buttons stay hidden.
 GITHUB_CLIENT_ID=PLACEHOLDER_GITHUB_OAUTH_CLIENT_ID
 GITHUB_CLIENT_SECRET=PLACEHOLDER_GITHUB_OAUTH_CLIENT_SECRET
 GITLAB_CLIENT_ID=PLACEHOLDER_GITLAB_CLIENT_ID