fix(manager-core): F-S-001 cap kv/docs/pubsub/queue payload sizes (default 256 KB)

Files (per-file cap), secrets (64 KB default), and email (25 MB default)
already enforce limits; kv::set, docs::create/update, pubsub::publish_durable
and queue::enqueue accepted any JSON value straight to a JSONB column with
no size validation. An anonymous public-HTTP script could fill disk via
queue::enqueue or amplify a single publish into N outbox rows × payload bytes.

Adds four new error variants:
- KvError::ValueTooLarge { limit, actual }
- DocsError::ValueTooLarge { limit, actual }
- PubsubError::MessageTooLarge { limit, actual }
- QueueError::PayloadTooLarge { limit, actual }

Each stateful service grows a `max_value_bytes` field with:
- Conservative 256 KB default (DEFAULT_KV_MAX_VALUE_BYTES etc.).
- New `with_max_*` constructor preserving the old `new()` signature.
- Env-knob reader (`*_max_*_from_env()`) — mirrors SecretsConfig::from_env.

Validation runs at the entry point BEFORE authz so an anonymous DoS doesn't
pay a membership lookup per attempt.

Wired via env knobs:
- PICLOUD_KV_MAX_VALUE_BYTES
- PICLOUD_DOCS_MAX_VALUE_BYTES
- PICLOUD_PUBSUB_MAX_MESSAGE_BYTES
- PICLOUD_QUEUE_MAX_PAYLOAD_BYTES

Documented in CLAUDE.md runtime config table.

New unit test in queue_service verifying the cap fires before authz.

AUDIT.md anchor: F-S-001. Depends on F-Q-004 (Backend variant).

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

crates/manager-core/src/pubsub_service.rs

@@ -70,6 +70,26 @@ fn load_i64(dst: &mut i64, key: &str) {
     }
 }
 
+/// Default per-message JSON-encoded payload cap (256 KB). Override with
+/// `PICLOUD_PUBSUB_MAX_MESSAGE_BYTES`.
+pub const DEFAULT_PUBSUB_MAX_MESSAGE_BYTES: usize = 256 * 1024;
+
+/// Read `PICLOUD_PUBSUB_MAX_MESSAGE_BYTES`; invalid values fall back to
+/// the conservative default with a warning.
+#[must_use]
+pub fn pubsub_max_message_bytes_from_env() -> usize {
+    if let Ok(v) = std::env::var("PICLOUD_PUBSUB_MAX_MESSAGE_BYTES") {
+        match v.trim().parse::<usize>() {
+            Ok(n) if n > 0 => return n,
+            _ => tracing::warn!(
+                value = %v,
+                "ignoring invalid PICLOUD_PUBSUB_MAX_MESSAGE_BYTES (want a positive integer)"
+            ),
+        }
+    }
+    DEFAULT_PUBSUB_MAX_MESSAGE_BYTES
+}
+
 pub struct PubsubServiceImpl {
     repo: Arc<dyn PubsubRepo>,
     authz: Arc<dyn AuthzRepo>,
@@ -80,6 +100,7 @@ pub struct PubsubServiceImpl {
     topics: Option<Arc<dyn TopicRepo>>,
     secrets: Option<Arc<dyn AppSecretsRepo>>,
     token_config: SubscriberTokenConfig,
+    max_message_bytes: usize,
 }
 
 impl PubsubServiceImpl {
@@ -92,9 +113,16 @@ impl PubsubServiceImpl {
             topics: None,
             secrets: None,
             token_config: SubscriberTokenConfig::conservative(),
+            max_message_bytes: DEFAULT_PUBSUB_MAX_MESSAGE_BYTES,
         }
     }
 
+    #[must_use]
+    pub fn with_max_message_bytes(mut self, n: usize) -> Self {
+        self.max_message_bytes = n;
+        self
+    }
+
     /// Attach the v1.1.6 realtime surface: the in-process broadcaster
     /// (publish fan-out to SSE subscribers), the topic registry +
     /// app-secrets repo (subscriber-token minting), and the TTL config.
@@ -142,6 +170,17 @@ impl PubsubService for PubsubServiceImpl {
         if topic.trim().is_empty() {
             return Err(PubsubError::EmptyTopic);
         }
+        // Reject oversized messages BEFORE the authz check so an
+        // anonymous public-script DoS doesn't pay the membership lookup.
+        let encoded_len = serde_json::to_vec(&message)
+            .map(|v| v.len())
+            .map_err(|e| PubsubError::Rejected(format!("encode message: {e}")))?;
+        if encoded_len > self.max_message_bytes {
+            return Err(PubsubError::MessageTooLarge {
+                limit: self.max_message_bytes,
+                actual: encoded_len,
+            });
+        }
         self.check_publish(cx).await?;
 
         // `published_at` is stamped once on the manager side so every