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/group_blobs_api.rs

@@ -14,7 +14,8 @@
 use std::sync::Arc;
 
 use axum::extract::{Path, Query, State};
-use axum::http::header::{CONTENT_LENGTH, CONTENT_TYPE};
+use axum::http::header::{CONTENT_DISPOSITION, CONTENT_LENGTH, CONTENT_TYPE};
+use axum::http::StatusCode;
 use axum::response::{IntoResponse, Json, Response};
 use axum::routing::get;
 use axum::{Extension, Router};
@@ -287,14 +288,29 @@ async fn get_file(
         .await
         .map_err(|e| GroupBlobsError::Backend(e.to_string()))?
         .ok_or(GroupBlobsError::NotFound)?;
-    Ok((
-        [
-            (CONTENT_TYPE, meta.content_type),
-            (CONTENT_LENGTH, bytes.len().to_string()),
-        ],
-        bytes,
-    )
-        .into_response())
+    // Same download hardening as the per-app path (`files_api::get_file`): a
+    // sanitized content-type, forced `attachment` disposition with a header-safe
+    // filename, plus nosniff/CSP so a stored HTML/SVG shared file can't render
+    // inline (stored-XSS) and no attacker-influenced byte can panic the builder.
+    let safe_ct = picloud_shared::sanitize_stored_content_type(&meta.content_type);
+    let disposition = format!(
+        "attachment; filename=\"{}\"",
+        picloud_shared::sanitize_stored_filename(&meta.name)
+    );
+    let len = bytes.len();
+    Response::builder()
+        .status(StatusCode::OK)
+        .header(CONTENT_TYPE, safe_ct)
+        .header(CONTENT_DISPOSITION, disposition)
+        .header(CONTENT_LENGTH, len)
+        .header("X-Content-Type-Options", "nosniff")
+        .header(
+            "Content-Security-Policy",
+            "default-src 'none'; sandbox; frame-ancestors 'none'",
+        )
+        .header("Referrer-Policy", "no-referrer")
+        .body(axum::body::Body::from(bytes))
+        .map_err(|e| GroupBlobsError::Backend(e.to_string()))
 }
 
 /// §11.6 D3: list a group's shared-queue dead-letters (newest-first). Guarded by