feat(pricing): expose CoinGecko /coins/list proxy endpoint

[TaprootFreak]

Context

Sister issue to #3712. The dfx-wallet app also calls CoinGecko directly to validate which on-chain tokens are listed on CoinGecko — used as a hard filter on dynamically-discovered tokens (only CoinGecko-listed tokens are rendered):

• https://api.coingecko.com/api/v3/coins/list?include_platform=true
• File: src/services/pricing/coingecko-coins-list.ts:18 (COINGECKO_LIST_URL)

Payload is ~2.6 MB / ~17 000 coins (Nov 2026). The wallet caches it client-side for 24 h, but every cold start hits CoinGecko directly — fingerprintable, and rate-limited under load.

Proposal

Add a public, unauthenticated endpoint mirroring the upstream shape:

GET /v1/pricing/coins-list?include_platform=true

Response identical to CoinGecko's coins/list (so the wallet only swaps the base URL):

[
  { "id": "bitcoin",  "symbol": "btc", "name": "Bitcoin",  "platforms": {} },
  { "id": "tether",   "symbol": "usdt","name": "Tether",   "platforms": { "ethereum": "0xdac17f95...", "polygon-pos": "0xc2132...", ... } },
  ...
]

Implementation notes

• Live next to PricingController (src/subdomains/supporting/pricing/pricing.controller.ts).
• Reuse CoinGeckoService / CoinGeckoClient — client.coinList({ include_platform: true }) returns the right shape.
• Server-side cache is essential here: response is ~2.6 MB and changes slowly. Cache for 24 h (matches what the wallet does locally and CoinGecko's own update cadence).
• Serve gzipped — the payload compresses ~10×.
• The pricing-proxy service on dfxprd/dfxdev already has CoinGecko caching and would also be a fine backing implementation.

Out of scope

• Filtering down the payload (e.g. only chains the wallet cares about). Keeping the upstream shape minimizes wallet diff and keeps the endpoint reusable.

Follow-up (wallet side)

Once live, the wallet swaps COINGECKO_LIST_URL in coingecko-coins-list.ts to \${DFX_API_URL}/v1/pricing/coins-list?include_platform=true and drops the direct CoinGecko call.