feat(event,storage): add events table substrate for OSS redesign

New internal/event package defines the Event type with canonical Kind
and Actor constants. New events table is appended via additive migration
(id, task_id, seq, ts, kind, actor, payload_json) with seq as per-task
monotonic, assigned by the storage layer inside a transaction.

CreateEvent + ListEvents support the future migration of question_json,
interactions_json, summary, and elaboration_input off the tasks table
into a unified, replayable event stream. No existing code paths are
modified yet; this is pure substrate for Phase 1.

https://claude.ai/code/session_01SESwn7kQ7oP62trWw6pc39

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