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

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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>

app/backend/internal/mcpserver/server.go

@@ -161,6 +161,10 @@ func init() {
 	register(spec{"containr_invites_list", "List team invite links (admin).", "GET", "/admin/invites", nil, nil, false})
 	register(spec{"containr_invites_create", "Create a team invite link (admin). Args: email (optional bind), expires_in_hours (default 168). Returns the one-time token + accept URL.", "POST", "/admin/invites", nil, []string{"email", "expires_in_hours"}, false})
 	register(spec{"containr_invites_revoke", "Revoke a team invite (admin). Args: id. Requires confirm=true.", "DELETE", "/admin/invites/{id}", []string{"id"}, nil, true})
+	register(spec{"containr_admin_impersonate", "Mint a 15-minute bearer token acting as a user (admin, audit-logged). Args: id (user id).", "POST", "/admin/users/{id}/impersonate", []string{"id"}, nil, false})
+	register(spec{"containr_github_app_status", "Show GitHub App provisioning status (admin).", "GET", "/admin/git/github-app", nil, nil, false})
+	register(spec{"containr_github_app_manifest", "Build a GitHub App manifest for self-provisioning (admin). Args: base_url (required, https public URL), name (optional), organization (optional).", "POST", "/admin/git/github-app/manifest", nil, []string{"base_url", "name", "organization"}, false})
+	register(spec{"containr_github_app_convert", "Exchange the GitHub manifest callback code for app credentials (admin). Args: code.", "POST", "/admin/git/github-app/convert", nil, []string{"code"}, false})
 
 	// Nodes / agent tokens (admin)
 	register(spec{"containr_nodes_list", "List node agents (admin)", "GET", "/agents", nil, nil, false})