Local recording (audio) management: playback, retention, disk usage

[YJack0000]

Manage the audio recordings tied to conversations. Audio is stored as files on disk, referenced by the conversation row (not blobbed into SQLite).

Scope

• Playback: play a conversation's recording, ideally seekable and synced to transcript lines where feasible.
• Storage controls: show where recordings live; per-conversation file size and total disk usage.
• Retention: option to keep or discard audio after a meeting; optional auto-delete policy (e.g. delete audio older than N days, keep transcript).
• Lifecycle integrity: deleting a conversation deletes its audio; deleting audio leaves the transcript intact and marks the conversation as "audio removed". No orphan files.
• Export: export/save the raw recording.
• Optional: a "recordings" overview showing disk usage with a way to bulk-clean.

Acceptance

• A user can play back a recording, see how much disk it uses, delete audio while keeping the transcript, and never end up with orphaned files.
• Retention policy (if set) removes audio as configured and is clearly surfaced in settings.

Notes

• This is the privacy/disk story for local data before any cloud option exists.

Part of #10.