feat(files): cache served files and add HTML text viewer

File cache:
- Add my_modules/filecache with metadata, blob, validator and store layers
- Cache convex.get_file via AsyncCache, TTL clamped to the file expiry
- Skip caching for missing, incomplete or already expired records
- Add a process-local, size-bounded LRU for immutable storage blobs
- Build the cache stack from FILE_CACHE_* env vars in build_file_cache
- Attach the cache to the app in my_modules/app/setup.py
- Expose app-bound helpers in filecache/access.py for routes

Serving:
- Send strong ETag, Last-Modified and Cache-Control on served files
- Answer If-None-Match and If-Modified-Since with 304 from metadata only
- Avoid pulling the blob from Convex on a revalidation hit
- Add a short Cloudflare-CDN-Cache-Control edge window for deleted files
- Record cache_status in the wide event on a 304
- Invalidate cached metadata after file update and delete in both routes

Text viewer:
- Add my_modules/textview to render text/plain and text/markdown as HTML
- Linkify http(s) URLs and trim trailing sentence punctuation
- Resolve links pointing at this instance to the referenced file
- Inline image previews, mark missing and expired targets
- Cap resolution at MAX_LOOKUPS backend lookups per document
- Serve the viewer only for real browser navigations, never for curl or CLI
- Keep raw bytes for ?raw=1, downloads and Accept headers without text/html
- Add templates/side/views/webpage/files/text_view.htm with a raw link

Other:
- Add tests for cache layers, serving, text rendering and the route
- Bump multidict to 7.2.1 in uv.lock
- Update the my_helpers submodule pointer

my_modules/filecache/access.py

@@ -0,0 +1,43 @@
+from __future__ import annotations
+
+from quart import current_app
+
+DEFAULT_BROWSER_MAX_AGE = 86400
+DEFAULT_EDGE_MAX_AGE = 1800
+
+def file_cache():
+  """The app's FileCache, or None when caching is not configured."""
+  return getattr(current_app, 'file_cache', None)
+
+def browser_max_age() -> int:
+  cache = file_cache()
+  return DEFAULT_BROWSER_MAX_AGE if cache is None else cache.blob_max_age
+
+def edge_max_age() -> int:
+  cache = file_cache()
+  return DEFAULT_EDGE_MAX_AGE if cache is None else cache.edge_max_age
+
+async def load_file(file_id: str):
+  async def loader():
+    return await current_app.convex.get_file(file_id=file_id)
+
+  cache = file_cache()
+  if cache is None:
+    return await loader()
+  return await cache.get_metadata(file_id, loader)
+
+async def load_blob(storage_key: str) -> bytes:
+  async def loader() -> bytes:
+    payload = await current_app.convex.get_from_storage(storage_key)
+    return payload.getvalue() if hasattr(payload, 'getvalue') else bytes(payload)
+
+  cache = file_cache()
+  if cache is None:
+    return await loader()
+  return await cache.get_blob(storage_key, loader)
+
+async def drop_file_cache(file_id: str) -> None:
+  """Invalidate metadata after a mutation; blobs are immutable and expire on TTL."""
+  cache = file_cache()
+  if cache is not None:
+    await cache.invalidate(file_id)