Working chat: auth, connections, streaming, folders

LLeMbas now runs end to end. Register, add an OpenAI-compatible
connection, and hold a real streaming conversation organised into
folders. Verified against the local llama-swap instance.

Streaming is the one genuinely tricky part. Sending a message returns
two HTML fragments -- the user bubble and an empty assistant bubble
carrying an sse-connect -- and that attribute is the ONLY thing that
starts a generation. Rendering an incomplete assistant message as a
streaming shell falls out of the same template, which means loading a
page whose last reply never finished simply picks it up again.

Details worth knowing about, each commented where it matters:

- SSE payloads are split across several data: lines. A raw newline in
  one data: line truncates the event, which shows up the first time a
  model emits a code block.
- Markdown is rendered server-side by the same helper for both the page
  and the final streamed frame, so the two cannot disagree. The fence
  renderer is replaced outright rather than using markdown-it's
  highlight option, which re-wraps output in a second <pre>.
- escape_text is html.escape, not nh3.clean_text: it escapes character
  by character, so escaping stream chunks separately equals escaping
  the whole string.
- The stream opens its own session via session_scope(); it outlives the
  request handler and the dependency-scoped session may be closed.
- Deleting a folder keeps the chats inside it (FK is SET NULL). Losing
  a conversation to a mis-clicked folder delete is unforgivable.
- Login failures use one message for "no such account" and "wrong
  password" so the form cannot enumerate registered addresses.

Also adds deploy/ for the gamebox install at https://chat.lan: system
unit, nginx vhost with buffering off (buffering on turns streaming into
one lump at the end), and install/update scripts following the same
service-user and /srv bind-mount conventions as llama-swap and comfyui.

70 tests, ruff clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

deploy/chat.lan.nginx.conf

@@ -0,0 +1,57 @@
+# chat.lan - HTTPS reverse proxy to LLeMbas (127.0.0.1:8080).
+# Deployed to /etc/nginx/conf.d/chat.lan.conf. Self-signed cert (chat.lan).
+#
+# Mirrors the comfy.lan and llama.lan vhosts on this box.
+
+server {
+    listen 80;
+    listen [::]:80;
+    server_name chat.lan;
+    return 301 https://$host$request_uri;
+}
+
+server {
+    listen 443 ssl;
+    listen [::]:443 ssl;
+    http2 on;
+    server_name chat.lan;
+
+    ssl_certificate     /etc/nginx/ssl/chat.lan.crt;
+    ssl_certificate_key /etc/nginx/ssl/chat.lan.key;
+    ssl_protocols       TLSv1.2 TLSv1.3;
+
+    # File uploads land here once that feature exists; 0 = no limit.
+    client_max_body_size 0;
+
+    location / {
+        proxy_pass http://127.0.0.1:8080;
+        proxy_http_version 1.1;
+        proxy_set_header Host              $host;
+        proxy_set_header X-Real-IP         $remote_addr;
+        proxy_set_header X-Forwarded-For   $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+
+        # Streamed replies are server-sent events. Every one of these matters:
+        # with buffering on, nginx holds the whole reply and delivers it in one
+        # lump at the end, which looks exactly like streaming being broken.
+        proxy_buffering         off;
+        proxy_request_buffering off;
+        proxy_cache             off;
+        # SSE is plain HTTP/1.1 chunked, so the connection header must not be
+        # the websocket upgrade dance -- it must simply stay open.
+        proxy_set_header Connection "";
+
+        # A model can think for minutes before the first token. The default
+        # 60s read timeout would cut long generations off mid-sentence.
+        proxy_read_timeout 3600s;
+        proxy_send_timeout 3600s;
+    }
+
+    # Static assets are immutable per release and never need revalidating.
+    location /static/ {
+        proxy_pass http://127.0.0.1:8080;
+        proxy_set_header Host $host;
+        expires 1h;
+        add_header Cache-Control "public";
+    }
+}