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/shared/src/pubsub.rs

@@ -73,6 +73,13 @@ pub enum PubsubError {
     #[error("pubsub rejected: {0}")]
     Rejected(String),
 
+    /// JSON-encoded message exceeded the configured `max_message_bytes`
+    /// cap (`PICLOUD_PUBSUB_MAX_MESSAGE_BYTES`). Defends against an
+    /// anonymous public script amplifying one publish into N outbox rows
+    /// × payload bytes.
+    #[error("pubsub: message too large ({actual} bytes; limit {limit})")]
+    MessageTooLarge { limit: usize, actual: usize },
+
     /// A `pubsub::subscriber_token` mint was rejected (empty topics,
     /// unregistered topic, ttl out of range, anonymous caller). The
     /// string is the full user-facing message; the SDK surfaces it