feat(cache): add chanora_cache crate with cacache-backed blob cache

- New chanora_cache crate: content-addressed blob store wrapping cacache
- BlobCache API: async put/get/remove/clear/total_size/evict
- Key validation: av_ prefix (32 hex chars), ic_ prefix (decimal digits)
- Cacache provides crash safety, SSRI integrity, content dedup
- Mtime-based eviction via cacache::list_sync + sort by timestamp
- 7 unit tests all passing
- Added to workspace members

crates/chanora_cache/Cargo.toml

@@ -0,0 +1,20 @@
+[package]
+name = "chanora_cache"
+description = "Chanora disposable content-addressed blob cache for avatars and icons"
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+authors.workspace = true
+license.workspace = true
+repository.workspace = true
+publish.workspace = true
+
+[dependencies]
+cacache = "13"
+thiserror.workspace = true
+tokio = { version = "1", features = ["fs", "rt"] }
+tracing.workspace = true
+
+[dev-dependencies]
+tempfile = "3"
+tokio = { version = "1", features = ["rt", "macros", "time"] }