feat(core): implement AuthContext, Identity, AuthToken, IdentityProvider, ConfigIdentityProvider (task: core/auth)

- auth.rs: Identity, AuthContext, AuthToken, IdentityProvider trait, ConfigIdentityProvider
- ConfigIdentityProvider reads from ArcSwap<DynamicConfig> on every call (hot-reloadable)
- Fingerprint resolution via authorized_fingerprints; token resolution via alk_ prefix + SHA-256 hash + expiry check
- config.rs: minimal DynamicConfig, AuthPolicy (with resolve methods), ApiKeyEntry, RateLimitConfig, ConfigReloadHandle required by auth
- Unit tests: fingerprint resolution (known/unknown/empty), token resolution (valid/expired/unknown/wrong-hash/non-alk), config reload changes results immediately
- Add sha2, hex deps to alknet-core

crates/alknet-core/Cargo.toml

@@ -29,4 +29,6 @@ tracing = "0.1"
 thiserror = "2"
 zeroize = { version = "1", features = ["alloc", "derive"] }
 bytes = "1"
-futures = "0.3"
+futures = "0.3"
+sha2 = "0.10"
+hex = "0.4"