fix: harden album buffer error paths

[QueryPlanner]

What

Hardens four latent robustness gaps in the Telegram photo-album buffering state machine (telegram/album_buffer.py and telegram/bot.py), found during a code-reduction audit and tracked as separate items in #163.

Why

None of the four gaps caused an observed failure, but two were "safe by accident": correct only because of how the code happens to be called or scheduled today, not because anything enforces it. A future refactor could silently reintroduce a stuck asyncio.Future (a hung Telegram turn) or a double-processed album. Fixing all four together keeps the album-buffer state machine self-consistent in one small, reviewable change rather than four disconnected diffs against the same file.

How

• _handle_album_turn now resolves album.future in its own finally block, so every validation early-return (too many photos, missing photo, oversized photo/album) is covered even if the method is ever called outside _process_flushed_album.
• AlbumBuffer.add_message now rolls back the self._buffers entry if asyncio.create_task for the max-wait watchdog raises, so a failed schedule can't leave an album with no timer able to flush it.
• The previously swallowed exception in _safe_handle_update (album-wait path) is now logged at warning instead of debug, with a comment explaining why it must not re-raise (the album turn already reports its own user-facing error).
• AlbumBuffer._flush now documents the synchronous, no-await invariant that keeps the debounce/max-wait race safe, so a future change that adds an await before album.processed = True doesn't silently reintroduce a double-flush.

Tests

• New test: _handle_album_turn resolves a pending album.future on a validation early-return, called directly (not via _process_flushed_album)
• New test: a create_task failure during add_message does not leave the album orphaned in self._buffers
• New test: the suppressed album-wait exception is logged at warning level
• New test: calling _flush twice back-to-back (simulating the debounce/max-wait race) flushes exactly once
• pytest tests/test_telegram_album_buffer.py tests/test_telegram_bot.py -q passes (245 passed)
• ruff check, ruff format --check pass with no new warnings
• mypy src/blacki/telegram/ passes with no new warnings

Related Issues

Closes #163

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