Track muted state on incidents

[Jan-Schuppik]

Previously the mute state (mute_reason) was stored on the object table, meaning all incidents for an object shared a single mute flag. This PR moves mute_reason onto the incident table so each incident independently owns its mute state.

• incident table gains a mute_reason column; IsMuted() now reads MuteReason.Valid directly on the incident instead of delegating to the object
• object table loses mute_reason; Object.IsMuted(), RestoreMutedObjects(), and the transient isMuted bool are all removed
• Schema migrations (0.3.0.sql) copy active mute state from object to any open incident before dropping the column, preserving mute state across upgrades
• handleMute/handleUnmute persist mute_reason via incident.Sync() in the same DB transaction as the history entry
• Tests add a makeIncidentFromEvent helper that drives the real ProcessEvent path, and a WithMutedIncidents sub-test in TestLoadOpenIncidents that asserts mute_reason survives a
  daemon reload

Resolves:

• Track muted state on incidents rather than objects #408