feat(git): GitHub App manifest self-provisioning + app-level webhook (#44)

* feat(branding): white-label — public /branding, admin write, shell theming

app_settings branding.* keys (product name, logo/favicon URLs, accent
color, docs/support links). Public GET /branding so login and shell
theme pre-auth; admin writes via PUT /settings.

useBranding() applies document.title, accent CSS var and favicon;
BrandWordmark replaces hardcoded wordmarks; admin Platform settings
gain a Branding form. MCP containr_branding_get + settings_update.

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

* feat(invites): team invite links — admin create/revoke, public accept

Admins mint single-use invite links (sha256-hashed token at rest,
optional email binding, configurable TTL). Public endpoints let the
accept page validate the link and register — working even while
public signup is disabled.

- migration + sqlc queries/model for user_invites
- POST /admin/invites (returns one-time token+url), GET list, DELETE revoke
- GET /auth/invites/:token public validation, POST /auth/accept-invite
- CLI containr invites list|create|revoke; MCP invites tools
- /auth/accept-invite page (expired/used/invalid states, bound-email lock)
- Admin page Team invites section: create, copy link, revoke
- OpenAPI + generated types + CLI guide

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

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

* feat(admin): impersonate user — 15-min token, audit-logged

POST /admin/users/:id/impersonate mints a short-lived bearer token
(impersonated_by claim) for support/debugging. Admin accounts cannot
be impersonated; every mint is audit-logged with actor + target.

- CLI: containr admin impersonate <user-id> (works via CONTAINR_TOKEN)
- MCP: containr_admin_impersonate
- Admin UI: per-user Impersonate action → token modal with usage hint
- OpenAPI + generated types

Verified live: token resolves to target, admin target 403, audit row.

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

* feat(git): GitHub App manifest self-provisioning + app-level webhook

Admins create the instance GitHub App in-browser instead of hand-
setting GITHUB_APP_* env vars:

- POST /admin/git/github-app/manifest builds the manifest + URL;
  the frontend auto-submits it to github.com
- /git/github-app/callback page exchanges the code via
  POST /admin/git/github-app/convert; credentials persist in
  app_settings (precedence over env)
- GET /admin/git/github-app reports configured/source
- POST /api/git/github-app/webhook — single app-level push receiver,
  HMAC-verified, dispatches via extracted dispatchPushToServices
- install-url + installation-token paths resolve settings→env
- CLI: containr admin github-app status|manifest|convert; MCP tools
- Git providers UI shows "Set up GitHub App" for admins when
  unconfigured

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

---------

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

app/backend/internal/api/routes.go

@@ -249,6 +249,10 @@ func SetupRoutes(router *gin.Engine, db *database.DB, redis *database.Redis, cfg
 	// replaces session auth. Providers POST to /api/git/webhooks/:id.
 	router.POST("/api/git/webhooks/:id", handleGitWebhookPush)
 
+	// GitHub App-level webhook: installed repos deliver to this single
+	// endpoint, verified against the provisioned webhook secret.
+	router.POST("/api/git/github-app/webhook", handleGitHubAppWebhook)
+
 	v1 := router.Group("/api/v1")
 	{
 		// Public routes (no authentication required)
@@ -477,6 +481,7 @@ func SetupRoutes(router *gin.Engine, db *database.DB, redis *database.Redis, cfg
 			admin.GET("/admin/overview", handleAdminOverview)
 			admin.GET("/admin/users", handleAdminListUsers)
 			admin.PATCH("/admin/users/:id", handleAdminSetUserAdmin)
+			admin.POST("/admin/users/:id/impersonate", handleAdminImpersonateUser)
 			admin.PATCH("/admin/projects/:id", handleAdminSetProjectApproval)
 
 			admin.POST("/users", handleCreateUser)
@@ -501,6 +506,11 @@ func SetupRoutes(router *gin.Engine, db *database.DB, redis *database.Redis, cfg
 			admin.PATCH("/admin/banners/:id", handleAdminUpdateBanner)
 			admin.DELETE("/admin/banners/:id", handleAdminDeleteBanner)
 
+			// GitHub App self-provisioning (manifest flow)
+			admin.GET("/admin/git/github-app", handleAdminGetGitHubApp)
+			admin.POST("/admin/git/github-app/manifest", handleAdminGitHubAppManifest)
+			admin.POST("/admin/git/github-app/convert", handleAdminGitHubAppConvert)
+
 			// Team invites — single-use registration links
 			admin.GET("/admin/invites", handleAdminListInvites)
 			admin.POST("/admin/invites", handleAdminCreateInvite)