feat(group-quota): per-group shared-collection quotas (M3)

Global env-var ceilings enforced in the group write path (mirrors the
per-value caps): PICLOUD_GROUP_KV_MAX_ROWS / _DOCS_MAX_ROWS (per-group row
count, checked only on a NEW key/doc — updates exempt) and
_FILES_MAX_TOTAL_BYTES (per-group total blob bytes). New group_quota env
helpers; count_rows/total_bytes repo methods (trait-default 0 for non-Postgres);
QuotaExceeded error variants on the three group error enums; a with_max_rows
test builder. Pinned by a group_kv_service unit test (3rd key rejected, update
of an existing key at the cap allowed).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/shared/src/group_docs.rs

@@ -176,6 +176,11 @@ pub enum GroupDocsError {
     #[error("group docs: data too large ({actual} bytes; limit {limit})")]
     ValueTooLarge { limit: usize, actual: usize },
 
+    /// The owning group's shared-docs store is at its row quota
+    /// (`PICLOUD_GROUP_DOCS_MAX_ROWS`); a new doc is rejected.
+    #[error("group docs: quota exceeded ({actual} rows; limit {limit})")]
+    QuotaExceeded { limit: usize, actual: usize },
+
     #[error("group docs backend error: {0}")]
     Backend(String),
 }