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

routes/api/link.py

@@ -19,6 +19,7 @@ from quart import current_app
 from my_modules.app.setup import LIMITER
 from my_modules.expiry import ensure_utc, parse_expires
 from my_modules.file_meta import format_size, iso_stamp_filename
+from my_modules.filecache.access import drop_file_cache
 from servicelink import Forbidden, InvalidParams, NotFound, Router, Unauthorized, any_verifier, bearer_verifier, create_link_blueprint, shared_secret_verifier
 
 MAX_RPC_BODY = 16 * 1024 * 1024
@@ -109,6 +110,7 @@ async def files_update(params, ctx):
     user_id=_user_id(ctx),
     preserve_missing=True,
   )
+  await drop_file_cache(file_id)
   return {'updated': True}
 
 @router.method('files.delete')
@@ -117,6 +119,7 @@ async def files_delete(params, ctx):
   if not file_id:
     raise InvalidParams('file_id is required')
   await current_app.convex.delete_file(file_id, _user_id(ctx))
+  await drop_file_cache(file_id)
   return {'deleted': True}
 
 async def _decode_access_token(token):