feat(reconcile): restore album shares after from-nextcloud-memories upload

[simonclausen]

This PR adds the new top-level immich-go reconcile workflow and its first source-specific implementation, reconcile nextcloud-memories, to complete the shared-album migration model for incremental multi-user Nextcloud Memories imports.

It is intended to work together with upload from-nextcloud-memories from #1370. The upload step preserves per-user ownership and writes migration state into Immich; the reconcile step later consumes that state to rebuild shared destination album membership for each participating user.

Closes #1365.

What changed

New reconcile workflow

• add top-level immich-go reconcile
• add immich-go reconcile nextcloud-memories
• add --cleanup-migration-tags to optionally remove consumed synthetic membership tags after successful reconciliation

Nextcloud Memories shared-album reconstruction

• read managed Nextcloud Memories album state embedded in destination album descriptions
• resolve synthetic membership tags of the form immich-go/src/nextcloud-memories/album/<source-album-id>
• list tagged assets for the authenticated user
• skip assets not owned by the authenticated user
• skip assets from external libraries
• add missing user-owned assets to the matching destination albums
• report malformed managed state, unresolved migration tags, permission failures, and albums with no matching user-owned assets

Workflow model

• keep upload and reconciliation as separate steps
• preserve ownership by having each user import into their own Immich account
• use Immich itself as the coordination surface for migration state
• support incremental convergence as different users migrate at different times

Documentation

• document the reconcile command and its intent
• add examples for the Nextcloud Memories reconciliation workflow
• update the command reference and main README
• remove temporary local planning docs now that the feature is ready for PR

Why

This addresses the shared album migration model in #1365 for the Nextcloud Memories migration flow introduced by #1366 / #1370.

The goal is to preserve the important user-facing relationships of a multi-user migration without centralizing ownership or relying on fragile local coordination files:

• albums stay under the correct owning Immich user
• assets stay in the correct user timeline
• shared destination albums can be reconstructed incrementally
• users can migrate independently and reconcile later

User impact

Users migrating from Nextcloud Memories can now:

1. import each user's own library with upload from-nextcloud-memories
2. preserve migration markers in Immich during import
3. run reconcile nextcloud-memories per user to add that user's imported assets into the corresponding shared destination albums

This is a post-import convergence step, not a continuous sync feature.

Current limitations

• reconciliation only considers assets owned by the authenticated Immich user
• external library assets are skipped
• cleanup only removes the specific synthetic album-membership tags used by reconciliation
• this does not attempt to preserve every source-specific sharing behavior from Nextcloud

Testing

Covered by focused tests for:

• command wiring and flag handling
• managed album state parsing
• matching tagged assets to destination albums
• skipping foreign-owned and external-library assets
• unresolved/malformed state handling
• cleanup of consumed migration tags
• propagation of list and cleanup errors