fix(security): server-authoritative approval gate + auth/session/file hardening

Remediate the HIGH and security-relevant findings from the 2026-07-11 audit.

H1 — the per-env approval gate is now server-authoritative. The governing
project is resolved from the target node's nearest-claimed ancestor
(`governing_env_policy`/`_tree` + `governing_project_id` +
`ProjectRepository::get_environments_by_id`), independent of the client-supplied
`[project]`. Omitting or spoofing the project block can no longer skip a gate the
owning project established; a to-create group resolves its declared parent's
chain so a fresh subtree node inherits the gate. Fails closed on any read error.

H2 — the API-key prefix slice (`&rest[..8]`) is now the boundary-safe
`rest.get(..8)`, so an attacker-supplied multibyte bearer can't panic the request
task (unauthenticated per-request DoS). Regression test added.

C1 — admin sessions gain an absolute lifetime cap (migration 0070,
`PICLOUD_SESSION_ABSOLUTE_TTL_HOURS`, default 30d): `lookup` filters it, `touch`
clamps the sliding bump to it, so a continuously-used or stolen-but-warm token
self-expires. Mirrors the data-plane app-user cap.

C2 — `Cache-Control: no-store` on the login and API-key-mint responses (the two
that return a raw credential), so a proxy/CDN/browser cache can't retain it.

B8 — file downloads are header-safe: `sanitize_stored_filename` guarantees a
valid `HeaderValue` (no panic on a control-char name) and BOTH the per-app and
group download paths now set attachment + `X-Content-Type-Options: nosniff` +
a restrictive CSP, closing a group-path stored-XSS gap.

Also folds in the server-side plan-warning plumbing (`plan_warnings`,
`PlanResult::warnings`) and the `app_only_reject` message helper that the CLI
plan-preview change builds on, plus operator security notes (reads-open shared-
topic SSE; the `--env` label is advisory, not a boundary).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/manager-core/src/api_keys_api.rs

@@ -18,7 +18,7 @@
 use std::sync::Arc;
 
 use axum::extract::{Path, State};
-use axum::http::StatusCode;
+use axum::http::{HeaderMap, StatusCode};
 use axum::response::{IntoResponse, Json, Response};
 use axum::routing::{delete, get};
 use axum::{Extension, Router};
@@ -116,7 +116,7 @@ async fn mint_key(
     State(state): State<ApiKeysState>,
     Extension(principal): Extension<Principal>,
     Json(input): Json<MintApiKeyRequest>,
-) -> Result<(StatusCode, Json<MintApiKeyResponse>), ApiKeysError> {
+) -> Result<(StatusCode, HeaderMap, Json<MintApiKeyResponse>), ApiKeysError> {
     validate_name(&input.name)?;
     validate_scopes(&input.scopes, input.app_id)?;
 
@@ -135,6 +135,8 @@ async fn mint_key(
         .await?;
     Ok((
         StatusCode::CREATED,
+        // C2: the response carries the raw key exactly once — never cache it.
+        crate::auth_api::no_store_headers(),
         Json(MintApiKeyResponse {
             key: row.into(),
             raw_token: minted.raw,