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/api/auth.go

@@ -546,6 +546,19 @@ func generateJWT(userID, email, secret string) (string, error) {
 	return token.SignedString([]byte(secret))
 }
 
+// generateImpersonationJWT mints a short-lived token acting as `userID`,
+// tagged with the admin who minted it so downstream callers can attribute
+// actions taken while impersonating.
+func generateImpersonationJWT(userID, email, actorID, secret string, expiresAt time.Time) (string, error) {
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, jwt.MapClaims{
+		"user_id":         userID,
+		"email":           email,
+		"impersonated_by": actorID,
+		"exp":             expiresAt.Unix(),
+	})
+	return token.SignedString([]byte(secret))
+}
+
 // ValidateJWT validates a JWT token and returns the claims
 func ValidateJWT(tokenString, secret string) (jwt.MapClaims, error) {
 	token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {