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/textview/linkify.py

@@ -0,0 +1,45 @@
+from __future__ import annotations
+
+import re
+
+# Only http(s) is linkified; everything else stays inert plain text.
+_URL_PATTERN = re.compile(r'https?://[^\s<>"\'`]+', re.IGNORECASE)
+
+# Prose punctuation that is almost never part of the URL itself.
+_TRAILING_PUNCTUATION = '.,;:!?\'"'
+_BRACKET_PAIRS = {')': '(', ']': '[', '}': '{'}
+
+def _trim_trailing(url: str) -> str:
+  """Give punctuation that belongs to the surrounding sentence back to the text."""
+  while url:
+    last = url[-1]
+    if last in _TRAILING_PUNCTUATION:
+      url = url[:-1]
+      continue
+    opening = _BRACKET_PAIRS.get(last)
+    if opening is not None and url.count(opening) < url.count(last):
+      url = url[:-1]
+      continue
+    break
+  return url
+
+def tokenize(text: str) -> list[tuple[str, str]]:
+  """Split text into ('text', value) and ('url', value) tokens, order preserved."""
+  tokens: list[tuple[str, str]] = []
+  cursor = 0
+
+  for match in _URL_PATTERN.finditer(text):
+    url = _trim_trailing(match.group(0))
+    if not url:
+      continue
+
+    start = match.start()
+    end = start + len(url)
+    if start > cursor:
+      tokens.append(('text', text[cursor:start]))
+    tokens.append(('url', url))
+    cursor = end
+
+  if cursor < len(text):
+    tokens.append(('text', text[cursor:]))
+  return tokens