fix(shared): F-Q-004 unify error variants — Forbidden on QueueError, rename Unavailable→Backend

Brings QueueError / PubsubError / InvokeError in line with sibling
shape used by KvError / DocsError / FilesError / SecretsError:

- QueueError gains an explicit Forbidden variant (previously authz
  denial was squashed into Rejected("forbidden") in queue_service.rs:68,
  losing the structured variant a 403-translation layer would need).
- QueueError::Unavailable renamed → Backend.
- PubsubError::Unavailable renamed → Backend.
- InvokeError::Unavailable renamed → Backend.
- Call sites updated (queue_service, pubsub_service, invoke_service,
  Noop* stubs, From<PubsubRepoError> impl, one test assertion).
- New unit test verifying authed-denied → QueueError::Forbidden.

AUDIT.md anchor: F-Q-004.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/shared/src/invoke.rs

@@ -105,9 +105,10 @@ pub enum InvokeError {
     #[error("invoke rejected: {0}")]
     Rejected(String),
 
-    /// Backend / database error.
+    /// Backend / database error. Named `Backend` to align with KvError /
+    /// DocsError / FilesError / SecretsError / QueueError / PubsubError.
     #[error("invoke backend error: {0}")]
-    Unavailable(String),
+    Backend(String),
 }
 
 /// Test-only stub: every call errors. Lets harnesses build a `Services`
@@ -122,7 +123,7 @@ impl InvokeService for NoopInvokeService {
         _cx: &SdkCallCx,
         _target: InvokeTarget,
     ) -> Result<ResolvedScript, InvokeError> {
-        Err(InvokeError::Unavailable("invoke is not wired in".into()))
+        Err(InvokeError::Backend("invoke is not wired in".into()))
     }
 
     async fn enqueue_async(
@@ -131,6 +132,6 @@ impl InvokeService for NoopInvokeService {
         _target: InvokeTarget,
         _args: serde_json::Value,
     ) -> Result<ExecutionId, InvokeError> {
-        Err(InvokeError::Unavailable("invoke is not wired in".into()))
+        Err(InvokeError::Backend("invoke is not wired in".into()))
     }
 }

crates/shared/src/pubsub.rs

@@ -52,7 +52,7 @@ pub trait PubsubService: Send + Sync {
         ttl_seconds: Option<i64>,
     ) -> Result<String, PubsubError> {
         let _ = (cx, topics, ttl_seconds);
-        Err(PubsubError::Unavailable(
+        Err(PubsubError::Backend(
             "subscriber tokens are not wired in".into(),
         ))
     }
@@ -80,9 +80,10 @@ pub enum PubsubError {
     #[error("{0}")]
     SubscriberToken(String),
 
-    /// Anything else — Postgres unavailable, etc.
+    /// Anything else — Postgres unavailable, etc. Named `Backend` to
+    /// align with KvError / DocsError / FilesError / SecretsError.
     #[error("pubsub backend error: {0}")]
-    Unavailable(String),
+    Backend(String),
 }
 
 /// Match a stored `topic_pattern` against a published `topic`.
@@ -145,7 +146,7 @@ impl PubsubService for NoopPubsubService {
         _topic: &str,
         _message: serde_json::Value,
     ) -> Result<(), PubsubError> {
-        Err(PubsubError::Unavailable("pubsub is not wired in".into()))
+        Err(PubsubError::Backend("pubsub is not wired in".into()))
     }
 }
 

crates/shared/src/queue.rs

@@ -61,6 +61,13 @@ pub enum QueueError {
     #[error("queue_name must not be empty")]
     EmptyName,
 
+    /// Caller principal lacked the required capability. Only raised when
+    /// `cx.principal.is_some()` (script-as-gate; public HTTP skips it).
+    /// Sibling shape across KvError / DocsError / PubsubError — uniform
+    /// for 403 translation at the HTTP boundary.
+    #[error("forbidden")]
+    Forbidden,
+
     /// Payload could not be serialized (e.g. a Rhai FnPtr / closure
     /// captured into the message). Rejected at the SDK boundary; surface
     /// the wording verbatim so scripts see the documented message.
@@ -71,9 +78,10 @@ pub enum QueueError {
     #[error("{0}")]
     InvalidOpts(String),
 
-    /// Backend unavailable (Postgres down, etc.).
+    /// Backend failure (Postgres unavailable, etc.). Named `Backend` to
+    /// align with KvError / DocsError / FilesError / SecretsError.
     #[error("queue backend error: {0}")]
-    Unavailable(String),
+    Backend(String),
 }
 
 /// Test-only stub: every call errors `Unavailable`. Used by harnesses
@@ -90,14 +98,14 @@ impl QueueService for NoopQueueService {
         _payload: serde_json::Value,
         _opts: EnqueueOpts,
     ) -> Result<QueueMessageId, QueueError> {
-        Err(QueueError::Unavailable("queue is not wired in".into()))
+        Err(QueueError::Backend("queue is not wired in".into()))
     }
 
     async fn depth(&self, _cx: &SdkCallCx, _queue_name: &str) -> Result<u64, QueueError> {
-        Err(QueueError::Unavailable("queue is not wired in".into()))
+        Err(QueueError::Backend("queue is not wired in".into()))
     }
 
     async fn depth_pending(&self, _cx: &SdkCallCx, _queue_name: &str) -> Result<u64, QueueError> {
-        Err(QueueError::Unavailable("queue is not wired in".into()))
+        Err(QueueError::Backend("queue is not wired in".into()))
     }
 }