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>

CLAUDE.md

@@ -120,6 +120,10 @@ Environment variables consumed by the `picloud` binary:
 | `PICLOUD_SANDBOX_MAX_*` | conservative defaults | Per-knob admin ceilings on Rhai sandbox overrides. See `manager-core::sandbox::SandboxCeiling`. |
 | `PICLOUD_FILES_ROOT` | `./data` | Filesystem root for `files::*` blob storage (v1.1.5). Bytes live at `<root>/files/<app_id>/<collection>/<id[0:2]>/<id>`; metadata in Postgres. |
 | `PICLOUD_FILES_MAX_FILE_SIZE_BYTES` | `104857600` (100 MB) | Per-file hard size cap for `files::*` (v1.1.5). Per-app quotas deferred to v1.2. |
+| `PICLOUD_KV_MAX_VALUE_BYTES` | `262144` (256 KB) | Per-key JSON-encoded value cap for `kv::set`. Rejects oversized payloads before authz so anonymous public scripts can't DoS Postgres JSONB columns. |
+| `PICLOUD_DOCS_MAX_VALUE_BYTES` | `262144` (256 KB) | Per-document JSON-encoded data cap for `docs::create`/`update`. |
+| `PICLOUD_PUBSUB_MAX_MESSAGE_BYTES` | `262144` (256 KB) | Per-message JSON-encoded payload cap for `pubsub::publish_durable`. Prevents one publish from amplifying into N outbox rows × M MB. |
+| `PICLOUD_QUEUE_MAX_PAYLOAD_BYTES` | `262144` (256 KB) | Per-message JSON-encoded payload cap for `queue::enqueue`. |
 
 ## Out of MVP