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/picloud/src/lib.rs

@@ -58,6 +58,10 @@ use tower_http::trace::TraceLayer;
 /// Default session TTL when `PICLOUD_SESSION_TTL_HOURS` isn't set.
 const DEFAULT_SESSION_TTL_HOURS: u64 = 24;
 
+/// Default absolute session lifetime cap (C1) when
+/// `PICLOUD_SESSION_ABSOLUTE_TTL_HOURS` isn't set — 30 days.
+const DEFAULT_SESSION_ABSOLUTE_TTL_HOURS: u64 = 30 * 24;
+
 /// Bundles the auth-related dependencies that both `build_app` and the
 /// startup bootstrap need. Built once in `main.rs` from the shared pool.
 pub struct AuthDeps {
@@ -65,6 +69,8 @@ pub struct AuthDeps {
     pub sessions: Arc<dyn AdminSessionRepository>,
     pub keys: Arc<dyn ApiKeyRepository>,
     pub ttl: Duration,
+    /// C1: absolute hard cap on an admin session (see `AuthState::absolute_ttl`).
+    pub absolute_ttl: Duration,
 }
 
 impl AuthDeps {
@@ -76,6 +82,7 @@ impl AuthDeps {
             sessions: Arc::new(PostgresAdminSessionRepository::new(pool.clone())),
             keys: Arc::new(PostgresApiKeyRepository::new(pool)),
             ttl: read_session_ttl(),
+            absolute_ttl: read_session_absolute_ttl(),
         }
     }
 }
@@ -89,6 +96,15 @@ fn read_session_ttl() -> Duration {
     Duration::from_secs(hours * 3600)
 }
 
+fn read_session_absolute_ttl() -> Duration {
+    let hours = std::env::var("PICLOUD_SESSION_ABSOLUTE_TTL_HOURS")
+        .ok()
+        .and_then(|s| s.parse::<u64>().ok())
+        .filter(|h| *h > 0)
+        .unwrap_or(DEFAULT_SESSION_ABSOLUTE_TTL_HOURS);
+    Duration::from_secs(hours * 3600)
+}
+
 /// Compose the manager + orchestrator routes on top of a shared
 /// Postgres pool, returning an Axum router ready to be served.
 ///
@@ -627,6 +643,7 @@ pub async fn build_app(
         sessions: auth.sessions.clone(),
         keys: auth.keys.clone(),
         ttl: auth.ttl,
+        absolute_ttl: auth.absolute_ttl,
         principal_cache: principal_cache.clone(),
         login_rate_limiter: Arc::new(
             picloud_manager_core::login_rate_limit::LoginRateLimiter::new(),