feat(files): fsspec backend so any filesystem can hold uploaded files

[daavoo]

Stacked on #976 (feat/files-api-parity); only the last commit is this PR.

What

files_backend: fsspec with files_url (gcs://bucket/prefix, abfs://container/prefix, s3://bucket/prefix, sftp://host/path, file:///path, memory://) and files_storage_options for the implementation's own keyword arguments (credentials, endpoints, projects). FsspecFileStore implements the same FileStore protocol as the local and boto3 S3 backends: writes and reads stream through worker threads, a partial upload is removed on failure or cancellation, backend exceptions are translated to the OSError family callers already handle, and a ref that could leave the root is refused.

fsspec was already in the dependency tree through any-llm and is now declared; the lock gains only that entry. files_storage_options is kept out of the settings view as a credential. Operators install the implementation package for their protocol (gcsfs, adlfs, s3fs, paramiko).

Validation

Unit tests run on fsspec's built-in memory:// and file:// filesystems, so no cloud package or network is needed. make lint, make typecheck, unit suite, OSS smoke gate, and the files/config/settings integration tests pass.

🤖 Generated with Claude Code

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[daavoo]

Folded into #1364 together with #976, rebased onto current main.