feat: send book thumbnails in Telegram notifications

Use Telegram sendPhoto API for conversion and metadata-approved events
when a book thumbnail is available on disk. Falls back to text message
if photo upload fails.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.lock

@@ -2285,6 +2285,7 @@ dependencies = [
  "base64",
  "bytes",
  "futures-core",
+ "futures-util",
  "http",
  "http-body",
  "http-body-util",
@@ -2293,6 +2294,7 @@ dependencies = [
  "hyper-util",
  "js-sys",
  "log",
+ "mime_guess",
  "percent-encoding",
  "pin-project-lite",
  "quinn",