[Enhancement]: SFTP source-tree collection can create a connection storm under directory concurrency #186

Problem

Current src/lib/adapters/storage/sftp.ts implements listTree() with breadth-first concurrent traversal backed by createConnectionPool({ limit: options?.concurrency ?? 1 }). With a wide tree and concurrency: 8, the pool may open eight SSH sessions merely to list the tree. On servers with strict handshake or connection limits, that can produce avoidable login storms and transient connection failures before any file transfer starts.

This is separate from SFTP file transfer session multiplexing (the openSession() path).

Proposal

Bound source-directory collection to one reusable SFTP/SSH connection while retaining concurrent directory requests on the SSH channel, a bounded retry only for transient handshake errors, and current abort/progress/path/exclusion/symlink behavior. Alternative: expose a configurable, low default connection ceiling while preserving a higher optional one for servers that need it. This requires maintainer input because upstream currently uses a connection pool by design.

Acceptance:

• listTree(..., {concurrency: 8}) opens no more than one SSH session in the proposed default mode while fetching multiple child directories correctly.
• No retry on authentication/unsupported-key errors, and at most one retry on timeout/reset during initial handshake.
• Abort and failure close the sole client; no leaked connections.
• Existing exclude, symlink and progress contracts remain unchanged.
• Tests reproduce the behavior and fail before implementation, then pass with the new one-connection mode.
• Source-only fix; no DB migration, deployment credentials or Homelab-specific configuration.

A downstream product currently carries this behavior and wants to converge upstream. A separate issue will cover file transfer session multiplexing rather than mixing this source-tree lifecycle with uploads/downloads.