Persistence on restart (stretch goal)

[n-lark]

Story 4 — Persistence on restart, stretch goal

As someone monitoring production, I want a restart not to wipe what my dashboard was showing, so that a deploy doesn't cost me the trend I was watching.

Everything above lives in memory, so refresh and reconnect work but a restart clears the store.

Acceptance criteria

• Snapshot the store's contents
• Rehydrate it on startup
• Tests covering restart

Open questions

• Whether this happens at all — snapshot the store and rehydrate on startup, or accept that a restart wipes it. This decision determines whether the three chart issues below can close.

Closes #1362, #1352, #753. Without this story, those stay open.

[cstns]

was the historian feature missed?

[n-lark]

was the historian feature missed?

We should discuss when we refine this @cstns