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/migrations/0070_admin_session_absolute_expiry.sql

@@ -0,0 +1,19 @@
+-- Audit 2026-07-11 (LOW C1): admin sessions had only a SLIDING expiry, so a
+-- continuously-used (or stolen-but-warm) admin token never self-expired. The
+-- data-plane app-user sessions (0027) already carry an absolute hard cap; give
+-- the higher-privilege admin session the same. `touch` clamps the sliding bump
+-- at this value, and `lookup` filters it, so an admin session dies at the later
+-- of its idle window or this absolute ceiling — whichever comes first.
+ALTER TABLE admin_sessions ADD COLUMN absolute_expires_at TIMESTAMPTZ;
+
+-- Backfill live rows: cap them 30 days from creation (the new default absolute
+-- TTL). A row already older than that will be swept on its next lookup.
+UPDATE admin_sessions
+   SET absolute_expires_at = created_at + INTERVAL '30 days'
+ WHERE absolute_expires_at IS NULL;
+
+ALTER TABLE admin_sessions ALTER COLUMN absolute_expires_at SET NOT NULL;
+
+-- Backs the absolute-expiry filter on lookup / prune.
+CREATE INDEX admin_sessions_absolute_expiry_idx
+    ON admin_sessions (absolute_expires_at);