Share a dashboard to a Snowflake role.

Snowflake-mode only (DVT-1670/DVT-1723): shares the dashboard to a Snowflake role — any caller who lives-holds that role gains read access. Requires share:manage on the dashboard (owner or a live dvt_admin; an owner whose LIVE tier is viewer is refused with 403 — DVT-4721, the owner needs dvt_editor or dvt_admin). Idempotent: sharing an already-shared role is a no-op. PUBLIC (and any other broadly-held pseudo-role) is rejected with 400 — sharing to PUBLIC would grant account-wide read, bypassing visibility: public. Outside snowflake mode this endpoint does not exist (404).

DVT-2260 (warn, don't block): the share is always created, but when the sharer's own live Snowflake role set resolves cleanly and does not contain the target role, the 201 body carries an advisory warning — the role may be misspelled or absent from the account (in which case the share grants nobody access), or simply a role the sharer does not hold, which is normal. The check is best-effort and fails open: no warning is returned when no live caller session is available or the resolve fails, so the absence of a warning is not proof the role exists.