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>

src/lembas/config.py

@@ -7,7 +7,7 @@ from functools import lru_cache
 from pathlib import Path
 from typing import Literal
 
-from pydantic import Field, field_validator
+from pydantic import Field, model_validator
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
 
@@ -22,6 +22,10 @@ class Settings(BaseSettings):
     )
 
     secret_key: str = Field(default="")
+    # Set when no LEMBAS_SECRET_KEY was supplied and one had to be invented.
+    # main.py warns about it at startup; see the validator below.
+    secret_key_is_ephemeral: bool = Field(default=False, exclude=True)
+
     data_dir: Path = Path("./data")
 
     host: str = "127.0.0.1"
@@ -34,13 +38,15 @@ class Settings(BaseSettings):
     session_ttl: int = 60 * 60 * 24 * 30
     request_timeout: float = 300.0
 
-    @field_validator("secret_key")
-    @classmethod
-    def _generate_secret_if_absent(cls, v: str) -> str:
-        # A generated key lets `lembas serve` work out of the box, but it changes
-        # on every restart: sessions drop and stored API keys become unreadable.
-        # main.py warns loudly about this. Never rely on it in production.
-        return v or secrets.token_urlsafe(48)
+    @model_validator(mode="after")
+    def _generate_secret_if_absent(self) -> Settings:
+        # A generated key lets `lembas serve` work with no configuration at all,
+        # but it changes on every restart: sessions drop and stored API keys
+        # become unreadable. Flagged so startup can warn. Never use in anger.
+        if not self.secret_key:
+            self.secret_key = secrets.token_urlsafe(48)
+            self.secret_key_is_ephemeral = True
+        return self
 
     @property
     def db_path(self) -> Path: