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/__init__.py

@@ -0,0 +1,27 @@
+from .blobs import BlobCache
+from .metadata import FileMetadataCache
+# access.py is imported directly by routes: it needs a Quart app context.
+from .store import FileCache, build_file_cache
+from .validators import (
+  build_etag,
+  cache_control,
+  edge_cache_control,
+  etag_matches,
+  is_fresh,
+  not_modified_since,
+  remaining_seconds,
+)
+
+__all__ = [
+  'BlobCache',
+  'FileCache',
+  'FileMetadataCache',
+  'build_etag',
+  'build_file_cache',
+  'cache_control',
+  'edge_cache_control',
+  'etag_matches',
+  'is_fresh',
+  'not_modified_since',
+  'remaining_seconds',
+]