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/kv_service.rs

@@ -25,10 +25,32 @@ use picloud_shared::{
 use crate::authz::{self, AuthzRepo, Capability};
 use crate::kv_repo::{KvRepo, KvRepoError};
 
+/// Default per-key JSON-encoded value cap (256 KB). Override with
+/// `PICLOUD_KV_MAX_VALUE_BYTES`.
+pub const DEFAULT_KV_MAX_VALUE_BYTES: usize = 256 * 1024;
+
+/// Read `PICLOUD_KV_MAX_VALUE_BYTES`; invalid values fall back to the
+/// conservative default with a warning. Public so the picloud binary can
+/// thread it through `KvServiceImpl::new`.
+#[must_use]
+pub fn kv_max_value_bytes_from_env() -> usize {
+    if let Ok(v) = std::env::var("PICLOUD_KV_MAX_VALUE_BYTES") {
+        match v.trim().parse::<usize>() {
+            Ok(n) if n > 0 => return n,
+            _ => tracing::warn!(
+                value = %v,
+                "ignoring invalid PICLOUD_KV_MAX_VALUE_BYTES (want a positive integer)"
+            ),
+        }
+    }
+    DEFAULT_KV_MAX_VALUE_BYTES
+}
+
 pub struct KvServiceImpl {
     repo: Arc<dyn KvRepo>,
     authz: Arc<dyn AuthzRepo>,
     events: Arc<dyn ServiceEventEmitter>,
+    max_value_bytes: usize,
 }
 
 impl KvServiceImpl {
@@ -37,11 +59,22 @@ impl KvServiceImpl {
         repo: Arc<dyn KvRepo>,
         authz: Arc<dyn AuthzRepo>,
         events: Arc<dyn ServiceEventEmitter>,
+    ) -> Self {
+        Self::with_max_value_bytes(repo, authz, events, DEFAULT_KV_MAX_VALUE_BYTES)
+    }
+
+    #[must_use]
+    pub fn with_max_value_bytes(
+        repo: Arc<dyn KvRepo>,
+        authz: Arc<dyn AuthzRepo>,
+        events: Arc<dyn ServiceEventEmitter>,
+        max_value_bytes: usize,
     ) -> Self {
         Self {
             repo,
             authz,
             events,
+            max_value_bytes,
         }
     }
 
@@ -102,6 +135,15 @@ impl KvService for KvServiceImpl {
         value: serde_json::Value,
     ) -> Result<(), KvError> {
         validate_collection(collection)?;
+        let encoded_len = serde_json::to_vec(&value)
+            .map(|v| v.len())
+            .map_err(|e| KvError::Backend(format!("encode value: {e}")))?;
+        if encoded_len > self.max_value_bytes {
+            return Err(KvError::ValueTooLarge {
+                limit: self.max_value_bytes,
+                actual: encoded_len,
+            });
+        }
         self.check_write(cx).await?;
         let previous = self
             .repo