feat(#12): add Redis caching layer with graceful fallback

[leanworld7-netizen]

Summary

Closes #12 — implements Redis caching layer for blockchain data queries with graceful fallback.

Changes

src/services/cache.js (new, ~200 lines):

• CacheService class wrapping ioredis with graceful degradation
• get(key) — returns parsed JSON or null on miss/error
• set(key, value, ttl) — stores with TTL via setex
• del(key) — deletes single key
• flush(pattern) — batch-deletes keys matching glob pattern (100-key batches)
• invalidate(entity, id) — invalidates all cache entries for an entity
• warm(entries) — pre-populates cache from fetcher functions
• health() — returns connection status + hit/miss/error stats + hit rate
• Key naming convention: equipchain:{entity}:{id}:{field}
• TTL presets: meter=60s, config=300s, analytics=120s, default=60s
• Retry strategy: exponential backoff, max 10 retries, capped at 2s
• Falls back to no-cache mode when Redis unavailable (no crashes)

src/middleware/cache.js (new, ~80 lines):

• cacheMiddleware(ttl) — Express middleware for automatic GET caching
• Only caches GET requests, varies key by auth status
• X-Cache: HIT|MISS and X-Cache-TTL response headers
• Auto-detects TTL from path (/config=300s, /analytics=120s, /meter=60s)
• invalidateCache(entity, id) — call after write operations

src/routes/index.js (modified):

• Added cache health stats to /health endpoint

tests/unit/cache.test.js (new, 15 tests):

• Tests all CRUD operations, TTL, flush, invalidate, warm, health
• Graceful fallback when Redis disconnected
• TTL preset values

package.json:

• Added ioredis ^5.4.0

Testing

• All 15 unit tests use a mock Redis client (no real Redis required)
• Graceful fallback tested explicitly
• Follows the spec's key naming convention and TTL defaults exactly