Previously the task logs endpoint only worked when the workspace was
running, leaving users unable to view task history after pausing.

This change adds snapshot retrieval with state-based branching: active
tasks fetch live logs from AgentAPI, paused/initializing/pending tasks
return stored snapshots (providing continuity during pause/resume), and
error/unknown states return HTTP 409 Conflict.

The response includes snapshot metadata (snapshot, snapshot_at) to
indicate whether logs are live or historical.

Closes coder/internal#1254