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/git.go

@@ -133,7 +133,9 @@ func handleGetGitProviders(c *gin.Context) {
 }
 
 func handleGetGitHubAppInstallURL(c *gin.Context) {
-	slug := strings.TrimSpace(os.Getenv("GITHUB_APP_SLUG"))
+	db, _ := c.Get("db")
+	d, _ := db.(*database.DB)
+	slug := strings.TrimSpace(settingValue(d, settingGitHubAppSlug, "GITHUB_APP_SLUG", ""))
 	if slug == "" {
 		c.JSON(http.StatusBadRequest, gin.H{"error": "GITHUB_APP_SLUG is not configured"})
 		return
@@ -1073,8 +1075,8 @@ func fetchBitbucketRepositories(providerAPIURL, accessToken string) ([]remoteGit
 }
 
 func getGitHubAppInstallationToken(baseURL string, installationID int64) (string, error) {
-	appID := strings.TrimSpace(os.Getenv("GITHUB_APP_ID"))
-	privateKeyPEM := strings.TrimSpace(os.Getenv("GITHUB_APP_PRIVATE_KEY"))
+	appID := platformSetting(settingGitHubAppID, "GITHUB_APP_ID")
+	privateKeyPEM := platformSetting(settingGitHubAppPrivateKey, "GITHUB_APP_PRIVATE_KEY")
 	if appID == "" || privateKeyPEM == "" {
 		return "", fmt.Errorf("GITHUB_APP_ID and GITHUB_APP_PRIVATE_KEY must be configured")
 	}