✨ feat(cache): add in-memory cache service (ADR-0022 Phase 1 part 2) (#23)

Phase 1 part 2 of ADR-0022 (companion to PR #22 rate-limit). In-memory cache service via go-cache, used by /api/version (60s TTL).

6/6 unit tests pass. ~95% Mistral autonomous via ICM workspace, cost €2.50 stages 01-02 (50% reduction vs T5 thanks to pre-extracted snippets in shared/).

Co-authored-by: Gabriel Radureau <arcodange@gmail.com>
Co-committed-by: Gabriel Radureau <arcodange@gmail.com>

config.yaml

@@ -87,4 +87,15 @@ database:
   
   # Maximum lifetime of connections (default: "1h")
   # Format: number + unit (s, m, h)
-  conn_max_lifetime: 1h
+  conn_max_lifetime: 1h
+
+# Cache configuration (in-memory)
+cache:
+  # Enable in-memory cache (default: true)
+  enabled: true
+  
+  # Default TTL in seconds for cache items (default: 300 = 5 minutes)
+  default_ttl_seconds: 300
+  
+  # Cleanup interval in seconds for expired items (default: 600 = 10 minutes)
+  cleanup_interval_seconds: 600