feat: implement cache backend with TTL support and integrate into con…

[pikann]

This pull request adds a Cache API for plugins — a TTL-based key-value store backed by the host's shared Valkey/Redis instance — alongside the existing DB/KV primitives, since neither of those can express "cache this for N minutes": DB is the shared Postgres, and KV is durable/non-expiring Postgres-backed storage.

New Cache API:

• Added CacheBackend interface and Cache type to backends.go: Get(key) (string, bool), Set(key, value string, ttl time.Duration), Delete(key). A zero TTL stores the value without expiry.
• Exposed via Context.Cache() (context.go), wired through newContext/NewContextForTest (context.go, testing.go) and the WASM dispatcher (dispatch.go).
• Implemented wasmCacheBackend in wasm_backends.go, backed by three new host imports declared in wasm_imports.go — paca.cache_get, paca.cache_set (with a ttlSeconds argument), and paca.cache_delete — mirroring the existing storage_get/storage_set/storage_delete (KV) trio.
• Added a stubCacheBackend in native_backends.go that always misses/no-ops, so non-WASM builds (go test, go vet) compile without a real host.
• Added InMemoryCache to plugintest/backends.go — a map-backed test double with a fake clock (Advance) so tests can assert TTL expiry without a real sleep — and exposed it as Context.Cache in plugintest/plugintest.go.
• Added plugintest/backends_test.go coverage: get-miss, set-then-get, zero-TTL-never-expires, expires-after-TTL, and delete.

Builds clean for both native (go build/go test) and GOOS=wasip1 GOARCH=wasm targets.

Follow-up (out of scope for this PR): the host-side implementation of paca.cache_get/set/delete — backed by Valkey/Redis and namespaced per plugin — is being wired up separately in the paca API service's plugin runtime. paca-plugin-dashboard is the first consumer, caching each panel's query result for 5 minutes (with immediate invalidation on panel edit/delete, and a bypass on manual reload) to avoid re-running potentially expensive aggregate queries on every dashboard render.