docs(adr): pluggable distributed cache (ADR 0002)

[jlc488]

Design-only PR (like #28 was for bootstrap). Captures the distributed-cache decision before any code.

The idea (your call from this session)

A consumer flips one property to turn the cache from in-memory to distributed (Redis), and never touches serialization. Their own @Cacheable methods just work across replicas.

devslab.kit.cache:
  type: in-memory   # none | in-memory | redis  (default in-memory)
  ttl: PT10M
  key-prefix: "devslab:"

Why

• Today's menu cache is a ConcurrentHashMap — breaks on >1 replica (stale on every node but the writer until TTL lapses).
• Every Spring Boot consumer otherwise re-solves distributed caching themselves, and the part that bites everyone is serialization (Serializable requirement, SerializationException, hand-configuring JSON + safe default-typing). A platform starter should decide that once.

What the ADR proposes

1. devslab.kit.cache.type switch → CacheManager bean (none / in-memory / redis), each conditionally wired; consumer's own CacheManager always wins.
2. Redis JSON serialization owned by the kit — GenericJackson2JsonRedisSerializer + a safe default-typing ObjectMapper (no LaissezFaireSubTypeValidator). This is the headline value.
3. New -cache-api / -cache-core modules; Redis stays optional & non-transitive (in-memory default keeps zero-dependency simple path).
4. The menu cache becomes the first consumer of the shared CacheManager — one cache story, and it stops being a multi-replica bug.
5. Consumers get distributed @Cacheable on their own code for free.

Plan (4 PRs)

cache-api/core + autoconfig → Redis path + serialization (real-Redis Testcontainers round-trip test) → migrate menu cache → sample-app(type=redis on the compose Redis it already starts) + docs.

Targeted for v0.1.0 (decided: cache first, then release).

Sign-off needed

1. Approve the type switch shape + property names.
2. Approve the 4-PR split.
3. OK to turn on @EnableCaching by default (guarded by @ConditionalOnMissingBean)?

Once you're happy I'll flip Status → Accepted and start PR 1.