0.2.26: security hardening from audit (P2/P3 fixes)

Address the security-auditor findings the user selected (items 2, 3, 4):

Default admin credentials (P2): remove the fixed `Ten31` default. First
boot now generates a strong random admin password (secrets.token_urlsafe),
records it 0600 at /data/.admin-password, and surfaces it once via a new
"Show Initial Admin Password" StartOS action (CLI `show-admin-password`).
The stored password is cleared when the admin is reset (CLI reset-password)
or self-changes it (change-password endpoint).

Login hardening (P2): add a per-IP in-memory sliding-window rate limiter
(10 failures / 5 min -> 429 + Retry-After) in ratelimit.py; run a dummy
argon2 verify when the user is unknown so timing can't enumerate usernames;
keep a single generic 401 for unknown-user and wrong-password.

Hardening (P3): server process now runs unprivileged -- Dockerfile adds
uid 10001 appuser; start.sh (still root) chowns the mounted /data then
drops via `setpriv` before exec'ing uvicorn. Spreadsheet imports are
size-capped via storage.read_capped (413 past MAX_UPLOAD_SIZE) in the
schedule, capital preview, and batch paths. batch_import no longer returns
raw exception text (generic per-file messages).

Verified in the packed amd64 container: PID1 uvicorn runs as uid 10001,
/data owned 10001 with 0600 secrets; generated admin password retrievable
via CLI and logs in (200); 11th bad login -> 429; admin reset clears the
stored password. Tests: test_auth_hardening.py (4). Full suite 21 passed;
frontend tsc + StartOS bundle clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/ten31portal/cli.py

@@ -1,10 +1,12 @@
 """CLI commands for Ten31Portal (also driven by the StartOS service Actions)."""
 
 import argparse
+import os
 import sys
 
 from sqlmodel import Session, select
 
+from ten31portal import config
 from ten31portal.auth import hash_password
 from ten31portal.database import engine
 from ten31portal.db_init import run_migrations
@@ -56,6 +58,14 @@ def create_user(args: argparse.Namespace) -> None:
         print(f"Created user: {user.name} ({user.username}) with role {user.role.value}")
 
 
+def _clear_admin_password_file() -> None:
+    """Remove the recorded initial admin password (it is no longer valid once changed)."""
+    try:
+        os.remove(config.ADMIN_PASSWORD_FILE)
+    except OSError:
+        pass
+
+
 def reset_password(args: argparse.Namespace) -> None:
     """Reset a user's password (and re-enable their login)."""
     run_migrations()
@@ -68,11 +78,29 @@ def reset_password(args: argparse.Namespace) -> None:
             sys.exit(1)
         user.password_hash = hash_password(args.password)
         user.login_enabled = True
+        is_admin = user.is_service_admin
         session.add(user)
         session.commit()
+        # Resetting the built-in admin invalidates the initial password recorded on first boot.
+        if is_admin:
+            _clear_admin_password_file()
         print(f"Password reset for {user.name} ({user.username}).")
 
 
+def show_admin_password(args: argparse.Namespace) -> None:
+    """Print the randomly-generated initial admin password recorded on first boot."""
+    path = config.ADMIN_PASSWORD_FILE
+    if not os.path.exists(path):
+        print(
+            "No stored initial password. It was either set explicitly at install time, or the "
+            "admin password has already been changed. Use Reset Password to set a new one."
+        )
+        return
+    with open(path) as f:
+        pw = f.read().strip()
+    print(pw if pw else "(the recorded initial password is empty)")
+
+
 def list_users(args: argparse.Namespace) -> None:
     """Print all user accounts."""
     run_migrations()
@@ -240,6 +268,8 @@ def main() -> None:
 
     sub.add_parser("list-users", help="List all user accounts")
 
+    sub.add_parser("show-admin-password", help="Show the initial admin password from first boot")
+
     delete = sub.add_parser("delete-user", help="Delete a user (not the Service Admin)")
     delete.add_argument("--username", required=False, default=None)
     delete.add_argument("--email", required=False, default=None)
@@ -261,6 +291,8 @@ def main() -> None:
         reset_password(args)
     elif args.command == "list-users":
         list_users(args)
+    elif args.command == "show-admin-password":
+        show_admin_password(args)
     elif args.command == "delete-user":
         delete_user(args)
     elif args.command == "dedupe-holdings":