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

templates/side/views/webpage/files/text_view.htm

@@ -0,0 +1,57 @@
+{% extends "base.htm" %}
+
+{% block title %}NanoShare - {{ file_name }}{% endblock %}
+
+{% block meta %}
+  <meta name="robots" content="noindex, nofollow" />
+{% endblock %}
+
+{% block head %}
+<style>
+  .text-view-page { padding: clamp(16px, 3vw, 28px); }
+  .text-view-head { display: flex; gap: 12px; flex-wrap: wrap; align-items: center; justify-content: space-between; }
+  .text-view-body {
+    margin: 16px 0 0;
+    padding: clamp(12px, 2vw, 18px);
+    border: 1px solid var(--border);
+    border-radius: 12px;
+    background: rgba(0, 0, 0, .16);
+    font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, monospace;
+    font-size: .95rem;
+    line-height: 1.6;
+    white-space: pre-wrap;
+    word-break: break-word;
+  }
+  .text-view-body a { color: var(--accent, #6cf); }
+  .text-view-ref { display: inline-flex; align-items: center; gap: 6px; vertical-align: middle; }
+  .text-view-ref img {
+    max-height: 120px;
+    max-width: 180px;
+    border-radius: 6px;
+    border: 1px solid var(--border);
+    vertical-align: middle;
+  }
+  .text-view-state { color: var(--muted); font-size: .85rem; }
+</style>
+{% endblock %}
+
+{% block content %}
+<main class="text-view-page">
+  <section class="card" style="padding: clamp(18px, 2.6vw, 28px);">
+    <div class="text-view-head">
+      <h1 class="page-title" style="margin:0;">{{ file_name }}</h1>
+      <a class="btn btn-ghost" href="{{ raw_url }}">View raw</a>
+    </div>
+
+    <div class="text-view-body">{% for segment in segments -%}
+{%- if segment.kind == 'text' -%}
+{{ segment.text }}
+{%- elif segment.kind == 'file' -%}
+<span class="text-view-ref"><a href="{{ segment.url }}" rel="noopener noreferrer">{{ segment.label }}</a>{% if segment.media == 'image' %} <a href="{{ segment.url }}" rel="noopener noreferrer"><img src="{{ segment.url }}" alt="{{ segment.label }}" loading="lazy"></a>{% endif %}</span>
+{%- else -%}
+<a href="{{ segment.url }}" rel="noopener noreferrer" target="_blank">{{ segment.label or segment.url }}</a>{% if segment.state == 'expired' %} <span class="text-view-state">(expired)</span>{% endif %}
+{%- endif -%}
+{%- endfor %}</div>
+  </section>
+</main>
+{% endblock %}