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/docs_service.rs

@@ -34,10 +34,31 @@ use crate::authz::{self, AuthzRepo, Capability};
 use crate::docs_filter::{parse_filter, FilterParseError};
 use crate::docs_repo::{DocsRepo, DocsRepoError};
 
+/// Default per-document JSON-encoded data cap (256 KB). Override with
+/// `PICLOUD_DOCS_MAX_VALUE_BYTES`.
+pub const DEFAULT_DOCS_MAX_VALUE_BYTES: usize = 256 * 1024;
+
+/// Read `PICLOUD_DOCS_MAX_VALUE_BYTES`; invalid values fall back to the
+/// conservative default with a warning.
+#[must_use]
+pub fn docs_max_value_bytes_from_env() -> usize {
+    if let Ok(v) = std::env::var("PICLOUD_DOCS_MAX_VALUE_BYTES") {
+        match v.trim().parse::<usize>() {
+            Ok(n) if n > 0 => return n,
+            _ => tracing::warn!(
+                value = %v,
+                "ignoring invalid PICLOUD_DOCS_MAX_VALUE_BYTES (want a positive integer)"
+            ),
+        }
+    }
+    DEFAULT_DOCS_MAX_VALUE_BYTES
+}
+
 pub struct DocsServiceImpl {
     repo: Arc<dyn DocsRepo>,
     authz: Arc<dyn AuthzRepo>,
     events: Arc<dyn ServiceEventEmitter>,
+    max_value_bytes: usize,
 }
 
 impl DocsServiceImpl {
@@ -46,14 +67,38 @@ impl DocsServiceImpl {
         repo: Arc<dyn DocsRepo>,
         authz: Arc<dyn AuthzRepo>,
         events: Arc<dyn ServiceEventEmitter>,
+    ) -> Self {
+        Self::with_max_value_bytes(repo, authz, events, DEFAULT_DOCS_MAX_VALUE_BYTES)
+    }
+
+    #[must_use]
+    pub fn with_max_value_bytes(
+        repo: Arc<dyn DocsRepo>,
+        authz: Arc<dyn AuthzRepo>,
+        events: Arc<dyn ServiceEventEmitter>,
+        max_value_bytes: usize,
     ) -> Self {
         Self {
             repo,
             authz,
             events,
+            max_value_bytes,
         }
     }
 
+    fn check_data_size(&self, data: &serde_json::Value) -> Result<(), DocsError> {
+        let encoded_len = serde_json::to_vec(data)
+            .map(|v| v.len())
+            .map_err(|e| DocsError::Backend(format!("encode doc data: {e}")))?;
+        if encoded_len > self.max_value_bytes {
+            return Err(DocsError::ValueTooLarge {
+                limit: self.max_value_bytes,
+                actual: encoded_len,
+            });
+        }
+        Ok(())
+    }
+
     async fn check_read(&self, cx: &SdkCallCx) -> Result<(), DocsError> {
         authz::script_gate(
             &*self.authz,
@@ -116,6 +161,7 @@ impl DocsService for DocsServiceImpl {
     ) -> Result<DocId, DocsError> {
         validate_collection(collection)?;
         validate_data(&data)?;
+        self.check_data_size(&data)?;
         self.check_write(cx).await?;
         let row = self
             .repo
@@ -193,6 +239,7 @@ impl DocsService for DocsServiceImpl {
     ) -> Result<(), DocsError> {
         validate_collection(collection)?;
         validate_data(&data)?;
+        self.check_data_size(&data)?;
         self.check_write(cx).await?;
         let previous = self
             .repo