Move MSTeams conversation and poll plugin-local stores to plugin-state SQLite. Legacy JSON stores import once without overwriting existing SQLite state; conversation and poll IDs are hashed for plugin-state keys; poll votes are sharded with bounded row-cap headroom and prune cleanup; MSTeams docs now describe SQLite storage. SSO and delegated token stores are unchanged. Verified with focused MSTeams tests, docs sanity, autoreview, Testbox check:changed, and green PR CI.
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