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/routers/import_router.py

@@ -16,6 +16,7 @@ import openpyxl
 from fastapi import APIRouter, Depends, File, Form, HTTPException, Query, UploadFile
 from sqlmodel import Session, select, col
 
+from ten31portal import storage
 from ten31portal.audit import record_audit
 from ten31portal.auth import require_role
 from ten31portal.database import get_session
@@ -342,7 +343,10 @@ def import_schedule(
     user: User = Depends(require_role(UserRole.approver, UserRole.cfo, UserRole.operations)),
     session: Session = Depends(get_session),
 ) -> dict[str, Any]:
-    file_bytes = file.file.read()
+    try:
+        file_bytes = storage.read_capped(file)
+    except storage.UploadTooLarge as exc:
+        raise HTTPException(status_code=413, detail=str(exc))
     filename = file.filename or ""
 
     # Parse the file into holdings/positions previews.