Fritz!: cache session in Settings instead of per-API connection

[andig]

Summary

Refactor: pull session caching out of the AHA and Smarthome connections and let `fritz.Settings.GetSessionID` own it. Both API clients now just call the shared helper instead of maintaining their own `SID`/`updated` fields and `refreshSession` plumbing.

Test plan

• `go build ./...`
• `go test ./meter/... ./charger/...`
• `go vet ./meter/fritz/... ./charger/...`
• Smoke-test a fritzdect (AHA) and fritzdect (smarthome) device live

[sourcery-ai]

Hey - I've found 1 issue, and left some high level feedback:

• Now that Settings caches sid/updated and is shared via pointers, GetSessionID introduces concurrent reads/writes to these fields across connections without synchronization; consider adding a mutex or other concurrency control around the cache to avoid data races.
• Changing GetSessionID to use a pointer receiver and internal mutable state assumes each logical Fritzbox configuration uses a single shared *Settings instance; double-check that other call sites don’t copy Settings by value in a way that would unintentionally bypass the shared session cache.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Now that `Settings` caches `sid`/`updated` and is shared via pointers, `GetSessionID` introduces concurrent reads/writes to these fields across connections without synchronization; consider adding a mutex or other concurrency control around the cache to avoid data races.
- Changing `GetSessionID` to use a pointer receiver and internal mutable state assumes each logical Fritzbox configuration uses a single shared `*Settings` instance; double-check that other call sites don’t copy `Settings` by value in a way that would unintentionally bypass the shared session cache.

## Individual Comments

### Comment 1
<location path="meter/fritz/types.go" line_range="24-25" />
<code_context>
 	URI, AIN, User, Password string
 	Firmware82               bool // use new REST API (FritzOS 8.2+)
+
+	sid     string
+	updated time.Time
 }

</code_context>
<issue_to_address>
**issue (bug_risk):** Accessing the cached SID and updated time via Settings may introduce data races if Settings is shared across goroutines.

Previously these values were per-connection, so contention wasn’t an issue. Now they’re shared via `*Settings`, so concurrent `GetSessionID` calls on the same instance will read/write `sid` and `updated` without synchronization, leading to races. If `Settings` can be used from multiple goroutines, please add appropriate synchronization (e.g., a mutex) or otherwise make `GetSessionID` concurrency-safe.
</issue_to_address>

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