fix: buffer Telegram photo-album messages into a single Claude request

[IliyaBrook]

Follow-up to the discussion in #186 — @MatveyF flagged that the same
"one logical message, N responses" problem happens with images:

if I attach images to my message I get a response per image.
E.g. if I send a piece of text with 3 images it will respond 3 times —
one for the text and the first image, second for the second image,
and third for the third image

Root cause

When a user sends a Telegram album (text + N photos), Telegram delivers
it as N separate Updates that share a common message.media_group_id;
the caption lives on only one of them. The agentic photo handler fires
once per Update, so Claude is invoked N times independently instead of
seeing the album as a single message.

Fix

• New MediaGroupBuffer (src/bot/utils/media_group_buffer.py) —
  debounces photos keyed by (user_id, chat_id, thread_id, media_group_id).
  Any photo carrying a media_group_id is buffered; a short timer
  (default 1.0s, configurable via MEDIA_GROUP_BUFFER_TIMEOUT, range
  0.3 – 5.0) fires after the last photo and flushes all photos +
  caption as a single payload.
• agentic_photo refactored into a dispatcher: standalone photos take
  the existing fast path; album photos go through the buffer.
• Shared work extracted to _process_photo_batch — the first photo is
  processed with the caption (so the prompt template keeps user intent),
  the rest are processed for image data only, and a single
  claude_integration.run_command is issued with all images attached.
• The Stop button cancels any pending media-group buffers for the user.