feat(diagnostics): add a shareable report without secrets or paths

GET /system/diagnostics builds the report on the server, off the event loop
and under a registry lease (409 busy while a restore runs): the project and
Viewer.js versions, Python and the platform, the four legacy checks (one
helper, legacy_checks, that GET /diagnostics also calls), public database
info, source counts, the log configuration and statistics, the last health
summary and the config.json-only keys from an explicit list. The CivitAI key
is reported only as has_civitai_api_key; every path is relative to ComfyUI
or reads "outside ComfyUI". A scrub replaces anything still absolute, the
key or the home folder and counts it in redacted (0 for a correct build).

The version parser moves unchanged to utils/version.py. (#329)

utils/version.py

@@ -0,0 +1,18 @@
+"""The project version, read from ``pyproject.toml`` (#329)."""
+
+import re
+from pathlib import Path
+
+
+def project_version(root):
+    """The version in ``root``'s ``pyproject.toml``; ``"unknown"`` on failure."""
+    try:
+        pyproject_path = Path(root) / "pyproject.toml"
+        if pyproject_path.exists():
+            content = pyproject_path.read_text()
+            match = re.search(r'version\s*=\s*["\']([^"\']+)["\']', content)
+            if match:
+                return match.group(1)
+    except Exception:  # nosec B110 - an unreadable file reads as unknown
+        pass
+    return "unknown"