feat(CORE-001D): close remaining boundary cleanup slices

docs/roadmap/done/CORE-001D2C-price-feed-cache-boundary.yaml

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+id: CORE-001D2C
+title: Price Feed Cache and Provider Normalization Boundary
+status: done
+priority: P2
+effort: S
+depends_on:
+  - CORE-001D2A
+tags:
+  - core
+  - decimal
+  - cache
+  - provider
+summary: PriceFeed now uses explicit cache/provider payload normalization instead of ad-hoc raw dict handling.
+completed_notes:
+  - Added explicit cached/provider payload normalization in `app/services/price_feed.py`.
+  - Removed the stale direct cache API usage and aligned `PriceFeed` with `CacheService.get_json(...)` / `set_json(...)`.
+  - Cached symbol mismatches and malformed cached payloads are discarded instead of being trusted.
+  - Added focused regression coverage in `tests/test_price_feed.py` for cache hits, malformed cache fallback, invalid provider payload rejection, and normalized cache writes.
+  - Validated with focused pytest coverage and `make build` on local Docker.