feat: add Redis-backed caching for external API calls

[Divineifed1]

close #58
summary:

Reduces redundant external API calls by caching token prices and Stellar balance queries in Redis.

Changes

quantara/web_app/contract_tools/cache.py (new): shared async Redis cache utility with lazy connection pool and graceful fallback on cache errors.
quantara/web_app/contract_tools/mixins/dashboard.py: DashboardMixin.get_current_prices() now cached at quantara:prices with 30s TTL.
quantara/web_app/contract_tools/blockchain_call.py: StellarClient.get_token_balances() now cached at quantara:balances:{address} with 60s TTL.
Impact

Removes N+1 per-user Horizon calls on each dashboard request.
Eliminates CoinGecko fetch on every dashboard refresh.
Cache is disabled-on-failure: Redis outage falls through to existing API fetch behavior with a warning log.
Notes for reviewers

Uses redis.asyncio from the already-pinned redis==8.0.0 dependency; no new packages.
REDIS_URL env var pattern matches existing usage in the rate limiter and health check.
Mock tests targeting APIRequest/get_token_balances should remain green because configuration runs through the patched names

[YaronZaki]

✅ Looks good, @Divineifed1. Clean separation between the cache helper and existing call sites, sensible TTLs (60s balances, 30s prices), and Redis is a nice pick. Merging.