Verify existing torrent data and allow seed-only adoption

[thsnkhn]

Problem

When a user adds a magnet link or torrent for files that already exist on disk but are not tracked by Harbor, Harbor starts downloading the payload again. The user cannot ask Harbor to verify the existing files and seed them.

Issue #60 fixed re-imports for torrents already known to Harbor. This issue covers untracked existing payloads.

Goal

Allow a user to select an existing destination, verify its torrent data, download only missing or invalid pieces, and seed immediately when all pieces are valid.

Acceptance criteria

• The add-torrent flow provides a clear option to use existing files.
• Harbor asks the torrent backend to verify all existing pieces before normal transfer begins.
• Valid files are not downloaded again.
• A fully valid payload enters completed or seeding state.
• A partial payload downloads only missing or invalid pieces.
• File selection is respected for multi-file torrents.
• Verification progress and failures are visible.
• Harbor does not overwrite existing data before verification.

Related

• Re-importing a completed torrent replaces its seeding entry and redownloads from zero #60
• Add torrent contents preview and file selection before start #15