feat: migrate to new backend logging convention

[AquiGorka]

Summary

Adopts the convention defined in local-dev/logging.md. Part of the 4-repo backend logging migration.

• New Logger at src/utils/logger/index.ts — TS port of AquiGorka/go-logger. Five methods (info, event, debug, error, scope), four levels.
• src/config/logger.ts exports createLogger(). No module-level singleton.
• Service-injection refactor: provider-auth, wallet-auth, payment-session, discord-notify free functions take { log } deps; scope internally.
• HTTP route handler factories: every handler (auth/challenge, auth/verify, account/, admin/councils/, pay/instant/*, utxo/post) becomes handle<X>(deps). Sub-routers (auth, account, admin, utxo, pay, waitlist) become buildXRouter(deps).
• Middleware factories: appendRequestIdMiddleware, adminMiddleware take { log }.
• All 28 LOG calls + 2 stray console.warn calls migrated. LOG.trace dropped (no equivalent).
• No OTEL changes — withSpan, span events, trace-context middleware preserved.

Logging coverage

Subsystem	Levels emitted
HTTP routes (auth, account, admin/councils, pay/instant, utxo)	info, event, debug, error
Middleware (requestId, admin auth)	event, debug, error
Services (wallet-auth, provider-auth, payment-session)	info, event, debug, error
External APIs (provider-platform, Discord)	info, event, debug, error
Error paths	error

Verified locally (pre-push)

• deno fmt --check: clean
• deno lint: clean
• deno task test:unit: 21 passed, 0 failed
• grep -rn 'LOG\.(...)' src/: 0 hits
• grep -rn 'console\.(...)' src/ (excl logger module): 0 hits

Test plan

• CI green
• Merge after local-dev PR #100 (convention doc) lands