feat(menu): wire cache-ttl property to a CachingMenuProvider decorator

[jlc488]

Follow-up #5/5 (and last of the backend wiring). #18 exposed devslab.kit.menu.cache-ttl-seconds on the Settings endpoint but the property wasn't actually wired to anything. This PR makes it real.

Property

DevslabKitProperties.Menu.cacheTtl (Duration, default PT5M). A zero or negative duration disables the cache decorator entirely — the right behaviour for tests that mutate menus or permissions and need the next read to see the change immediately.

CachingMenuProvider

New menu-core decorator wrapping any MenuProvider:

• Per-user keying (user.id()) — the delegate's MenuTree is already permission-filtered, so the result varies per user even inside the same tenant. Per-user keys keep the cache correct without invalidation on tenant-wide menu edits; the TTL handles that.
• ConcurrentHashMap with per-entry expiry timestamps. Lazy eviction on read, no background thread, no LRU bound. Plenty for "a few dozen admins per tenant, menus loaded once per session" — swap in Caffeine if the access pattern outgrows it.
• Manual invalidate(userId) and invalidateAll() exposed for admin tooling that mutates menus and wants the next read to skip the cache.
• Inject Clock so tests can advance time without sleeping.

Wiring (autoconfigure)

MenuAutoConfiguration.menuProvider:

• Builds the database-backed DefaultMenuProvider as before.
• Wraps it in CachingMenuProvider when devslab.kit.menu.cache-ttl is positive (default 5m).
• Skips the decorator when ttl <= 0 — returns the bare DefaultMenuProvider.
• @EnableConfigurationProperties added so the new Duration field binds correctly from configuration sources.

Test plan

• ./gradlew build green (55 tasks; sample-app context loads, all tests pass)
• Manual: set devslab.kit.menu.cache-ttl=PT0S, hit /menus admin endpoint, change a menu, hit /menus/tree from the UI → see the change immediately. Then set PT5M, repeat, confirm the change is delayed up to 5m or until invalidate*() is called.

Series wrap

This was the last of the five backend follow-ups. The full back-half of the contract-gap series:

#	Topic	PR
1	outcome / ip / userAgent first-class on AuditEvent	#19
2	PolicyEvaluation richer decision (reason + matchedRules)	#20
3	PermissionGrant path on Diagnostics matchedVia	#21
4	async-queue-capacity wired to a bounded executor	#22
5	cache-ttl wired to a per-user caching decorator	this PR

The admin-ui's 9 CRUD pages still ship with hardcoded English labels — i18n migration of those pages is the last open item; landing it on the UI repo separately.