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/capital_import_router.py

@@ -19,6 +19,7 @@ import openpyxl
 from fastapi import APIRouter, Depends, File, Form, HTTPException, UploadFile
 from sqlmodel import Session, select
 
+from ten31portal import storage
 from ten31portal.audit import record_audit
 from ten31portal.auth import hash_password, require_internal_admin
 from ten31portal.database import get_session
@@ -195,7 +196,11 @@ def preview_import(
     if entity_id is not None and session.get(Entity, entity_id) is None:
         raise HTTPException(status_code=404, detail="Entity not found")
 
-    wb = _open_workbook(file.file.read(), password)
+    try:
+        file_bytes = storage.read_capped(file)
+    except storage.UploadTooLarge as exc:
+        raise HTTPException(status_code=413, detail=str(exc))
+    wb = _open_workbook(file_bytes, password)
 
     investors = session.exec(select(User).where(User.role == UserRole.investor)).all()
     by_name = {u.name.strip().lower(): u for u in investors}
@@ -388,7 +393,7 @@ def batch_import(
         fname = upload.filename or "(unnamed)"
         res = BatchCapitalFileResult(filename=fname)
         try:
-            wb = _open_workbook(upload.file.read(), password)
+            wb = _open_workbook(storage.read_capped(upload), password)
             if "ALLOC SI" not in wb.sheetnames:
                 raise HTTPException(status_code=422, detail="No ALLOC SI tab found in this workbook.")
             as_of, roster = _parse_alloc_si(wb)
@@ -452,11 +457,15 @@ def batch_import(
             session.commit()
             total_statements += res.statements_written
         except HTTPException as e:
+            # Our own controlled messages (bad password, no ALLOC SI, no date) are safe to show.
             session.rollback()
             res.error = e.detail
-        except Exception as e:  # noqa: BLE001 — surface any parse failure per-file, keep going
+        except storage.UploadTooLarge:
             session.rollback()
-            res.error = str(e) or "Could not process this file."
+            res.error = "File is too large."
+        except Exception:  # noqa: BLE001 — keep going on any parse failure; don't leak internals
+            session.rollback()
+            res.error = "Could not process this file (unexpected format or error)."
         results.append(res)
 
     return BatchCapitalImportResult(