feat: add subscribe command for plan management and discount coupons

[gulshngill]

Summary

• New command: nansen subscribe with subcommands: plans, promo-code, create, cancel, status
• Subscription backend: Targets app.nansen.ai via new appBaseUrl on NansenAPI (env: NANSEN_APP_URL)
• Payment providers: Stripe (default), Coinbase, Moonpay — with optional --promo-code for discount coupons

New files

• src/commands/subscribe.js — command handler, table formatters
• src/__tests__/subscribe.test.js — 20 unit tests (help, plans, promo-code, create, cancel, status, formatters)

Modified files

• src/api.js — appBaseUrl property, appRequest() helper, 7 new API methods
• src/cli.js — import + register subscribe command, table formatting for subscribe subcommands, HELP string
• src/schema.json — subscribe command schema

Test plan

• npm test passes (all 24 test files, 1299 tests)
• Manual test: nansen subscribe plans --table
• Manual test: nansen subscribe promo-code TESTCODE
• Manual test: nansen subscribe create --price-id <id> --promo-code <code>
• Manual test: nansen subscribe status
• Manual test: nansen subscribe cancel