Add Cloudflare R2 file library storage backend

Implement R2FileStore, a FileStore backed by Cloudflare R2 via its
S3-compatible API, selectable with --file-store r2 and configured through
R2_* environment variables.

The store follows the MemFileStore model: a flat keyspace where directories
are derived from key prefixes, with zero-byte marker objects so empty folders
persist, and errors.ErrUnsupported for symlink/alias operations. Because R2
has no append, in-progress .incomplete uploads are routed to a local staging
directory (real O_APPEND and size-based resume) and promoted to a finished R2
object on the terminal upload-commit Rename; all other paths live in R2.

A narrow s3API seam plus an s3Uploader interface make the backend unit-testable
against an in-memory fake without a network. Adds a user setup guide at
docs/r2-file-store.md.

1 files changed, 10 insertions, 0 deletions

diff --git a/README.md b/README.md
index 3165535..3a25a5b 100644
--- a/README.md
+++ b/README.md
@@ -170,6 +170,16 @@ Usage of mobius-hotline-server:
 
 To run as a systemd service, refer to this sample unit file: [mobius-hotline-server.service](https://github.com/jhalter/mobius/blob/master/cmd/mobius-hotline-server/mobius-hotline-server.service)
 
+## File Storage
+
+By default the file library is stored on the server's local disk. The `-file-store` flag selects an alternate backend:
+
+- `os` (default) — local filesystem
+- `memory` — in-memory, non-persistent (for testing)
+- `r2` — [Cloudflare R2](docs/r2-file-store.md) object storage
+
+See [docs/r2-file-store.md](docs/r2-file-store.md) for R2 setup instructions.
+
 ## HTTP API
 
 The Mobius server includes an optional HTTP API for server administration and user management.