feat(merchants): add merchant discovery and location management

[ryzen-xp]

• Close Real P2P merchant discovery — replace hardcoded offers #73

What was implemented

Backend (micopay/backend)

src/routes/merchants.ts — added 3 new endpoints:

• GET /merchants/available?lat&lng&radius_km&amount_mxn&flow — public, returns merchants sorted by haversine distance, filtered by amount range and availability
• PATCH /merchants/me/location — authenticated, lets a merchant set their lat/lng/address
• GET /merchants/me/trades — authenticated, returns the merchant's trade history (was already called from frontend but missing from backend)

src/services/merchant.service.ts — added:

• getAvailableMerchants() — haversine SQL query with LEAST(1.0, ...) guard to prevent acos domain errors, filters by merchant_available, location not null, amount within min/max, distance within radius
• Updated getOrCreateMerchantConfig and updateMerchantConfig to include latitude, longitude, address_text fields

sql/migrations/002_merchant_discovery.sql — creates merchant_configs table (idempotent) and adds location columns with ALTER TABLE … ADD COLUMN IF NOT EXISTS

Frontend (micopay/frontend)

src/services/api.ts — added:

• getMerchantsAvailable(query) — calls GET /merchants/available
• updateMerchantLocation(token, lat, lng, address) — calls PATCH /merchants/me/location
• AvailableMerchant and MerchantsAvailableQuery types

src/hooks/useMerchantsAvailable.ts — new hook that:

• Requests geolocation (Capacitor-native + browser fallback)
• Handles permission denied → location_denied state
• Fetches from the real API
• Returns typed MerchantsState union: idle | loading | location_denied | error | empty | success
• Exposes refetch() for retry

src/pages/ExploreMap.tsx — fully rewritten:

• Calls useMerchantsAvailable with flow: 'cashout'
• DEFAULT_OFFERS array removed entirely
• Loading skeleton, location-denied state, network error state, empty state, and success state all handled
• onSelectOffer(offer.id) now passes the real seller_id UUID

src/pages/DepositMap.tsx — fully rewritten:

• Calls useMerchantsAvailable with flow: 'deposit'
• "Tienda Don Pepe" and "Usuario @ana_m" hardcoded cards removed
• Same state machine as ExploreMap
• onSelectOffer passes real seller_id

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