fc-common: add disk space monitoring and error handling

Adds a `DiskSpace` error variant and an `is_disk_full()` helper
alongside a `check_disk_space()` util that uses libc's `statsfs` on
unix. FC now detects disk space errors and logs recovery instructions
if applicable.

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: Ia6d2c472219dc9d6eed6901dc733d7bb6a6a6964

crates/common/Cargo.toml

@@ -14,6 +14,7 @@ clap.workspace               = true
 config.workspace             = true
 git2.workspace               = true
 hex.workspace                = true
+libc.workspace = true
 lettre.workspace             = true
 regex.workspace              = true
 reqwest.workspace            = true