feat(booth): view .md (rendered) and .txt/.log in-booth without downloading

Loose .md/.txt/.log files rendered as forced-download links in the gallery
and downloaded (or showed raw) when opened. Now they open in a readable
in-booth page via the existing /b/<name>/view route:
  - .md  -> rendered HTML (Python-Markdown: fenced code, tables, sane lists),
           styled in an Australis .markdown-body with the viewer chrome;
  - .txt/.log -> preformatted <pre> text view.
The gallery links docs to the viewer (📄) instead of a download; the view
page keeps a ⬇ (?dl=1) for saving. Files over 2 MB hand back raw. New
markdown dep (optional-import: degrades .md to text view if absent).
booth_image_view -> booth_view_file (now handles image + doc + raw-fallback).
9 new tests, suite 44 passing; deployed + verified live on nh3-dev :8090.

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "booth"
-version = "0.1.5"
+version = "0.1.6"
 description = "The Booth — a dead-simple standing web server that scans a data dir of drop-folders and renders each as an ephemeral media 'booth' (image/webm/audio auto-gallery, or a folder's own index.html verbatim). Also accepts browser/curl uploads for pickup under a human-readable id. 24h TTL, then the folder is wiped. Fleet tool for CC sessions to surface A/B and smoke results to the operator."
 requires-python = ">=3.11"
 dependencies = [
@@ -8,6 +8,7 @@ dependencies = [
     "uvicorn[standard]>=0.34",
     "jinja2>=3.1",
     "python-multipart>=0.0.9",
+    "markdown>=3.5",
 ]
 
 [project.optional-dependencies]