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/backend/internal/api/auth.go

@@ -120,13 +120,31 @@ func handleAuthBootstrap(c *gin.Context) {
 		mode = "register"
 	}
 
+	providers := []string{}
+	for _, provider := range []string{"GITHUB", "GOOGLE"} {
+		if oauthProviderConfigured(provider) {
+			providers = append(providers, strings.ToLower(provider))
+		}
+	}
+
 	c.JSON(http.StatusOK, gin.H{
 		"has_users":  count > 0,
 		"user_count": count,
 		"mode":       mode,
+		"providers":  providers,
 	})
 }
 
+// oauthProviderConfigured reports whether an OAuth provider has real
+// credentials. Placeholder values from .env.example do not count, so
+// self-hosted installs default to email/password only.
+func oauthProviderConfigured(provider string) bool {
+	id := strings.TrimSpace(os.Getenv(provider + "_CLIENT_ID"))
+	secret := strings.TrimSpace(os.Getenv(provider + "_CLIENT_SECRET"))
+	return id != "" && secret != "" &&
+		!strings.HasPrefix(id, "PLACEHOLDER_") && !strings.HasPrefix(secret, "PLACEHOLDER_")
+}
+
 type betterAuthLoginUser struct {
 	ID    string `json:"id"`
 	Email string `json:"email"`