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>

src/app_helpers.py

@@ -1,6 +1,13 @@
 # src/app_helpers.py
-import os
 import base64
+import logging
+import os
+
+from fastapi import HTTPException
+from fastapi.responses import HTMLResponse
+from starlette.requests import Request
+
+logger = logging.getLogger(__name__)
 
 def read_if_exists(path: str) -> str:
     """Read file if it exists, return empty string otherwise."""
@@ -20,6 +27,28 @@ def abs_join(base_dir: str, rel: str) -> str:
     """Join paths and return absolute path."""
     return os.path.abspath(os.path.join(base_dir, rel))
 
+def serve_html_with_nonce(request: Request, file_path: str) -> HTMLResponse:
+    """Read an app-bundled HTML page and inject the CSP nonce into inline <script> tags.
+
+    Callers pass fixed, server-owned template paths (index/login/backgrounds),
+    never a client-supplied path. So any read failure here — a missing file
+    (broken deployment) or a permission/IO error — is a server fault, not a
+    client "not found": map all of them to a logged 500 so a missing core
+    template surfaces in 5xx alerting instead of hiding behind a 404. If a
+    future caller serves a client-influenced path where 404 is correct, branch
+    that at the call site rather than defaulting this shared helper to 404.
+    """
+    try:
+        with open(file_path, "r", encoding="utf-8") as f:
+            html = f.read()
+    except OSError:
+        logger.exception("Failed to read page %s", file_path)
+        raise HTTPException(500, "Internal server error")
+    nonce = getattr(request.state, "csp_nonce", "")
+    html = html.replace("{{CSP_NONCE}}", nonce)
+    return HTMLResponse(html)
+
+
 def inside_base_dir(base_dir: str, path: str) -> bool:
     """Check if path is inside base directory."""
     if not isinstance(base_dir, str) or not isinstance(path, str):