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/auth_router.py

@@ -1,15 +1,27 @@
 """Authentication endpoints."""
 
+import os
+import secrets
+
 from fastapi import APIRouter, Depends, HTTPException, Request
 from sqlmodel import Session, select
 
+from ten31portal import config
 from ten31portal.auth import get_current_user, hash_password, verify_password
 from ten31portal.database import get_session
 from ten31portal.models import User
+from ten31portal.ratelimit import SlidingWindowLimiter
 from ten31portal.schemas import ChangePasswordRequest, LoginRequest, UserResponse
 
 router = APIRouter(prefix="/api/auth", tags=["auth"])
 
+# Throttle password guessing per client IP: at most 10 failed attempts per 5 minutes.
+_login_limiter = SlidingWindowLimiter(max_attempts=10, window_seconds=300)
+
+# A throwaway hash verified when no user matches, so a missing account costs the same argon2
+# time as a real one — otherwise the timing difference reveals which usernames exist.
+_DUMMY_HASH = hash_password(secrets.token_urlsafe(16))
+
 
 @router.post("/login")
 def login(
@@ -17,13 +29,30 @@ def login(
     request: Request,
     session: Session = Depends(get_session),
 ) -> UserResponse:
+    client_ip = request.client.host if request.client else "unknown"
+    retry_after = _login_limiter.retry_after(client_ip)
+    if retry_after > 0:
+        raise HTTPException(
+            status_code=429,
+            detail="Too many login attempts. Please wait a moment and try again.",
+            headers={"Retry-After": str(int(retry_after) + 1)},
+        )
+
     # Accept either a username or an email in the login field.
     handle = body.login.strip()
     user = session.exec(select(User).where(User.username == handle)).first()
     if user is None:
         user = session.exec(select(User).where(User.email == handle)).first()
-    if user is None or not verify_password(body.password, user.password_hash):
+    # Always run a verify (dummy hash when the user is unknown) so success/failure take the
+    # same time, and give one generic message so neither branch leaks whether the user exists.
+    if user is None:
+        verify_password(body.password, _DUMMY_HASH)
+        _login_limiter.record_failure(client_ip)
         raise HTTPException(status_code=401, detail="Invalid username or password")
+    if not verify_password(body.password, user.password_hash):
+        _login_limiter.record_failure(client_ip)
+        raise HTTPException(status_code=401, detail="Invalid username or password")
+    # Past this point the password was correct, so these messages don't aid guessing.
     if user.primary_account_id is not None:
         raise HTTPException(
             status_code=401,
@@ -33,6 +62,7 @@ def login(
         raise HTTPException(status_code=401, detail="This account does not have a login yet.")
     if not user.is_active:
         raise HTTPException(status_code=401, detail="Account disabled")
+    _login_limiter.reset(client_ip)
     request.session["user_id"] = user.id
     return UserResponse.model_validate(user, from_attributes=True)
 
@@ -62,4 +92,10 @@ def change_password(
     user.password_hash = hash_password(body.new_password)
     session.add(user)
     session.commit()
+    # Once the built-in admin sets their own password, the initial one from first boot is stale.
+    if user.is_service_admin:
+        try:
+            os.remove(config.ADMIN_PASSWORD_FILE)
+        except OSError:
+            pass
     return {"status": "ok"}