fix(routes): log and cleanly 500 on unreadable HTML page (#4637)

* fix(routes): serve 404 instead of 500 when an HTML page file is missing

_serve_html_with_nonce opened the HTML file with no error handling, and
callers such as /backgrounds and /login pass their paths in with no
existence check, so a missing or unreadable file raised an unhandled
OSError that surfaced as a 500. Wrap the read and raise HTTPException(404)
instead; the normal render path (CSP-nonce substitution) is unchanged.

Fixes #4594

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

* fix(routes): distinguish missing page (404) from read failure (500)

The previous fix caught a broad OSError and returned 404 for every
failure, which masks real server-side problems (permission errors, I/O
failures) as "not found" and lets them slip past error alerting. Split
FileNotFoundError (genuine 404) from other OSError, which now logs the
exception and returns a generic 500 — without leaking the OS error
string or file path into the response body.

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

* fix(routes): treat unreadable bundled HTML page as logged 500, not 404

Per PR #4637 review: every caller of the page-render helper serves a fixed,
server-owned template (index/login/backgrounds), never a client-supplied
path. So a missing or unreadable file is a server fault (broken deployment),
not a client "not found" — a 404 there mislabels a server error and hides a
missing core template from 5xx alerting, contradicting the OSError->500
rationale this PR is built on. Collapse both branches into a single logged,
leak-free 500.

Move the helper to src.app_helpers.serve_html_with_nonce so the behavior can
be unit-tested without importing the whole app (app.py is the slim
orchestrator; the test harness stubs src.database, so importing app in tests
is not viable). Add tests pinning missing/unreadable -> 500 (not 404) and
nonce injection on the happy path.

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

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

app.py

@@ -44,7 +44,7 @@ from typing import Dict
 
 from contextlib import asynccontextmanager
 from fastapi import FastAPI, Request, HTTPException
-from fastapi.responses import JSONResponse, FileResponse, HTMLResponse
+from fastapi.responses import JSONResponse, FileResponse
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.staticfiles import StaticFiles
 from starlette.middleware.base import BaseHTTPMiddleware
@@ -65,7 +65,7 @@ from core.exceptions import (
 
 import bcrypt as _bcrypt
 
-from src.app_helpers import abs_join
+from src.app_helpers import abs_join, serve_html_with_nonce
 from src.generated_images import GENERATED_IMAGE_HEADERS, resolve_generated_image_path
 from starlette.responses import RedirectResponse
 
@@ -791,22 +791,14 @@ app.include_router(setup_companion_routes())
 
 # ========= ROUTES (kept in app.py) =========
 
-def _serve_html_with_nonce(request: Request, file_path: str) -> HTMLResponse:
-    """Read an HTML file and inject the CSP nonce into inline <script> tags."""
-    with open(file_path, "r", encoding="utf-8") as f:
-        html = f.read()
-    nonce = getattr(request.state, "csp_nonce", "")
-    html = html.replace("{{CSP_NONCE}}", nonce)
-    return HTMLResponse(html)
-
 @app.get("/")
 async def serve_index(request: Request):
     static_path = abs_join(BASE_DIR, "static/index.html")
     if os.path.exists(static_path):
-        return _serve_html_with_nonce(request, static_path)
+        return serve_html_with_nonce(request, static_path)
     root_path = abs_join(BASE_DIR, "index.html")
     if os.path.exists(root_path):
-        return _serve_html_with_nonce(request, root_path)
+        return serve_html_with_nonce(request, root_path)
     raise HTTPException(404, "index.html not found")
 
 @app.get("/notes")
@@ -848,13 +840,13 @@ async def serve_library(request: Request):
 @app.get("/backgrounds")
 async def serve_backgrounds(request: Request):
     """Sandbox page for prototyping background effects. No auth required."""
-    return _serve_html_with_nonce(request, abs_join(BASE_DIR, "static/backgrounds.html"))
+    return serve_html_with_nonce(request, abs_join(BASE_DIR, "static/backgrounds.html"))
 
 @app.get("/login")
 async def serve_login(request: Request):
     if not AUTH_ENABLED:
         return RedirectResponse(url="/", status_code=302)
-    return _serve_html_with_nonce(request, abs_join(BASE_DIR, "static/login.html"))
+    return serve_html_with_nonce(request, abs_join(BASE_DIR, "static/login.html"))
 
 @app.get("/api/version")
 async def get_version():