Close the read-side privilege escalation and fix proxy-aware IPs (0.44.0)

F1 — Any authenticated user could read any file the backend could see.
/api/files/read and /download hung on get_current_user, and the sandbox that
should have caught that was open by default: ALLOWED_BROWSE_ROOTS contained
"/", for which _is_allowed() waves through every path. So the `user` role could
download stackpilot.db (password hashes, agent tokens, backup credentials),
every stack's .env and every .secrets/* file — with no audit trail, because
only mutations were logged.

Implementing that turned up three more doors into the same room, all fixed
here since closing only the first would have made the fix cosmetic:
GET /api/stacks/{id} handed the .env to any user, /export tarred the whole
stack dir including .secrets/*, and both the agent file proxies and
/api/agents/{id}/stacks/{id} repeated the leak for every remote host. All 24
filesystem-touching routes are now admin-only; reads and downloads are audited
(listing is not — the Files page polls it). DATA_DIR is refused outright, since
the API deliberately masks agent tokens and destination secrets and the browser
would otherwise be the way around that. "/" is out of the default browse roots.

F2 — Backup destination credentials were plaintext JSON in the DB, which is
what made F1 worth exploiting. They are now Fernet-encrypted at rest behind
parse_config/dump_config, with existing rows migrated at startup.

This needed a prerequisite from F6: the key is derived from SECRET_KEY, which
was regenerated on every boot when unset. Encrypting against a key that changes
per restart would be worse than plaintext, so an auto-generated SECRET_KEY is
now persisted to ${DATA_DIR}/secret_key at mode 0600. Sessions surviving a
restart is a welcome side effect.

F3 — /api/audit is admin-only. Also hidden from the dashboard and the nav for
non-admins, so nobody polls into a 403.

F4 — uvicorn now runs with --proxy-headers, so nginx's X-Forwarded-For is
honoured. Without it request.client.host was the frontend container's IP for
every request, which made the login rate limit global instead of per-IP (10
failures locked out everyone) and filled the audit log's IP column with one
useless value.

Verified: encrypt/decrypt round-trip incl. plaintext passthrough, idempotent
re-encryption and wrong-key handling; sandbox denial for DATA_DIR, traversal
into it, and paths outside the roots, with the allowed roots still reachable.
Both against stubbed settings — there is no Docker here, so nothing was run
end to end.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Dk43rmEeRfYi5wsLDbmfyG

backend/services/backup_destination_service.py

@@ -1,8 +1,11 @@
 """Push/pull stack backups to remote destinations (SFTP, S3-compatible or NFS).
 
 All operations are synchronous (paramiko / boto3 / docker); async callers
-should wrap them with ``asyncio.to_thread``. Destination config is a plain
-dict parsed from the ``BackupDestination.config`` JSON column.
+should wrap them with ``asyncio.to_thread``. Destination config is a dict
+stored in the ``BackupDestination.config`` column as JSON, encrypted at rest
+(:mod:`services.crypto_service`) because it carries SFTP passwords, SSH keys
+and S3 secret keys. Always go through :func:`parse_config` / :func:`dump_config`
+— never touch the column directly.
 """
 from __future__ import annotations
 
@@ -17,6 +20,7 @@ import tempfile
 from typing import Any
 
 from models.backup_destination import BackupDestination
+from services import crypto_service
 
 logger = logging.getLogger("stackpilot.backup_dest")
 
@@ -26,12 +30,48 @@ class DestinationError(Exception):
 
 
 def parse_config(dest: BackupDestination) -> dict:
+    """Decrypt and parse a destination's config.
+
+    Tolerates plaintext (pre-encryption rows) and returns ``{}`` rather than
+    raising if the value can't be decrypted — a destination whose key is gone
+    should show up as unconfigured in the UI, not take the whole list down with
+    a 500. The failure is logged with the destination name so it's findable.
+    """
     try:
-        return json.loads(dest.config or "{}")
+        raw = crypto_service.decrypt(dest.config or "{}")
+    except crypto_service.DecryptError as exc:
+        logger.error("Destination '%s': %s", dest.name, exc)
+        return {}
+    try:
+        return json.loads(raw or "{}")
     except json.JSONDecodeError:
         return {}
 
 
+def dump_config(config: dict) -> str:
+    """Serialise and encrypt a config dict for storage."""
+    return crypto_service.encrypt(json.dumps(config or {}))
+
+
+def migrate_plaintext_configs(session) -> int:
+    """Encrypt destination configs written before encryption existed.
+
+    Runs once at startup. Returns how many rows were rewritten.
+    """
+    from sqlmodel import select
+
+    migrated = 0
+    for dest in session.exec(select(BackupDestination)).all():
+        if crypto_service.is_encrypted(dest.config):
+            continue
+        dest.config = crypto_service.encrypt(dest.config or "{}")
+        session.add(dest)
+        migrated += 1
+    if migrated:
+        session.commit()
+    return migrated
+
+
 # --------------------------------------------------------------------------- #
 # SFTP (paramiko)
 # --------------------------------------------------------------------------- #