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.

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

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.

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

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/admin.go

@@ -246,6 +246,57 @@ func handleAdminSetUserAdmin(c *gin.Context) {
 	c.JSON(http.StatusOK, gin.H{"message": "User updated"})
 }
 
+// POST /admin/users/:id/impersonate — mints a 15-minute bearer token acting
+// as the target user. Admin-only; admin accounts cannot be impersonated
+// (would blur audit attribution). Every use is audit-logged at mint time;
+// token carries `impersonated_by` for downstream attribution.
+func handleAdminImpersonateUser(c *gin.Context) {
+	db := c.MustGet("db").(*database.DB)
+	targetID := strings.TrimSpace(c.Param("id"))
+	if _, err := uuid.Parse(targetID); err != nil {
+		respondError(c, http.StatusBadRequest, "INVALID_ID", "invalid user id")
+		return
+	}
+
+	var email, name string
+	var isAdmin bool
+	err := db.QueryRowContext(c.Request.Context(),
+		`SELECT email, COALESCE(name, ''), is_admin FROM users WHERE id = $1`, targetID).
+		Scan(&email, &name, &isAdmin)
+	if err != nil {
+		respondError(c, http.StatusNotFound, "NOT_FOUND", "user not found")
+		return
+	}
+	if isAdmin {
+		respondError(c, http.StatusForbidden, "FORBIDDEN", "admin accounts cannot be impersonated")
+		return
+	}
+	actorID := optionalUserID(c)
+	if actorID == targetID {
+		respondError(c, http.StatusBadRequest, "VALIDATION", "cannot impersonate yourself")
+		return
+	}
+
+	expiresAt := time.Now().UTC().Add(15 * time.Minute)
+	jwtSecret := c.MustGet("jwt_secret").(string)
+	token, err := generateImpersonationJWT(targetID, email, actorID, jwtSecret, expiresAt)
+	if err != nil {
+		respondError(c, http.StatusInternalServerError, "INTERNAL", "failed to generate token")
+		return
+	}
+
+	LogAuditWithRequest(c, "user", targetID, "impersonate", map[string]interface{}{
+		"actor_id":    actorID,
+		"target_email": email,
+		"expires_at":  expiresAt.Format(time.RFC3339),
+	})
+	c.JSON(http.StatusOK, gin.H{
+		"token":      token,
+		"expires_at": expiresAt.Format(time.RFC3339),
+		"user":       gin.H{"id": targetID, "email": email, "name": name},
+	})
+}
+
 func handleAdminSetProjectApproval(c *gin.Context) {
 	db := c.MustGet("db").(*database.DB)
 	queries := sqlcdb.New(db.DB)