perf(ui): cache busting — HTML no-cache + versioned asset URLs (?v=) with immutable 1y asset caching

Phase 33 (story: .agent/user_stories/cache-busting.md).

- app/core/caching.py: asset_version() — git short SHA (a commit is a
  deploy), stable content-hash fallback for non-git checkouts, "dev"
  for a missing static dir; computed once per process. CachingMiddleware
  — the five HTML pages revalidate (no-cache) with ?v=<token> asset refs
  rewritten in flight; /assets/* is public, max-age=31536000, immutable;
  everything else (all /api/*, the SSE chat stream in particular) passes
  through byte-identical.
- tests/e2e/test_cache_busting.py: fresh-Chromium wire assertions —
  document no-cache, versioned CSS/JS request URLs sharing one token,
  immutable asset headers, /api/health baseline headers, SSE chat to
  done (mock LLM).
- README 'Caching / deploys' section + story file.

Also fixed two prod-image defects surfaced by this phase's podman smoke
(the full app would not boot):
- Containerfile: ship the scripts/ package — app/api/sync.py (phase 32)
  imports scripts.git_sync / scripts.import_docs at module level, so the
  container crashed on boot (ModuleNotFoundError: No module named
  'scripts').
- compose.yaml: pass BOR_ADMIN_PASSWORD / BOR_SESSION_SECRET through to
  the app service (:- defaults keep 'podman compose up -d db' working;
  the app's own fail-loud gate still names missing admin auth).

Smoke: podman compose --profile prod up -d on a fresh image + a fresh
Chromium profile — /, /sources.html and /login.html all served
Cache-Control: no-cache; all 8 asset requests versioned with one shared
token (content-hash fallback inside the image — no .git there);
/assets/* immutable for a year.

tests/unit/test_main.py

@@ -19,3 +19,14 @@ def test_create_app_warns_and_serves_api_only_without_static_dir(
     assert client.get("/api/health").status_code == 200
     # …but the static mount is absent (no index page).
     assert client.get("/").status_code == 404
+
+
+def test_create_app_wires_caching_middleware() -> None:
+    """Phase 33: the app factory always attaches the cache-busting
+    middleware (by name) — even when the static dir is missing, so the
+    /api/* no-touch guarantee holds in every environment."""
+    app2 = main_mod.create_app()
+    # ``mw.cls`` is starlette's opaque ``_MiddlewareFactory`` protocol —
+    # reach for ``__name__`` the same way starlette's own __repr__ does.
+    middleware_names = [getattr(mw.cls, "__name__", "") for mw in app2.user_middleware]
+    assert "CachingMiddleware" in middleware_names