server: track UI-created device lifetime cancel in sync.Map

[DerAndereAndi]

Devices created or updated via the config UI now register their cancel function in a package-level sync.Map keyed by config ID. deleteDevice loads and fires the cancel; updateDevice swaps old for new so the previous instance's cancel is invoked after the new one is installed. This gives resource-holding devices (for example EEBus chargers and meters) a clean hook to release external state via their existing <-ctx.Done() goroutine — no new public interface, no changes to configurableDevice.

Known limitation: devices loaded at startup from the database (via cmd/setup.go:configurableInstance) are not registered in the map. Deleting such a device through the UI will not trigger its lifetime cancel, so any external registration will linger until evcc restart. The common create-via-UI → delete-via-UI flow is fully covered. Closing this gap requires the per-device lifecycle work that is not yet implemented.

[sourcery-ai]

Hey - I've left some high level feedback:

• The package-level deviceCancels sync.Map with raw any values and repeated .(context.CancelFunc) assertions would be easier to reason about and safer if you wrapped it in small helper functions (e.g. setDeviceCancel, swapDeviceCancel, cancelAndDeleteDevice) so the casting and lifecycle semantics live in one place.
• The long limitation comment above deviceCancels mixes behavioural rationale with future plans; consider shortening it and moving the detailed explanation into a higher-level design note or the PR description to keep this handler file focused on implementation details.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The package-level `deviceCancels` `sync.Map` with raw `any` values and repeated `.(context.CancelFunc)` assertions would be easier to reason about and safer if you wrapped it in small helper functions (e.g. `setDeviceCancel`, `swapDeviceCancel`, `cancelAndDeleteDevice`) so the casting and lifecycle semantics live in one place.
- The long limitation comment above `deviceCancels` mixes behavioural rationale with future plans; consider shortening it and moving the detailed explanation into a higher-level design note or the PR description to keep this handler file focused on implementation details.

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