feat: Stripe secrets pipeline — env wiring, client refactor, health check, k6 test

[GTC6244]

Summary

• Wire STRIPE_SECRET_KEY and STRIPE_PUBLISHABLE_KEY into docker-compose.phala.yml as Phala CVM encrypted env vars (same ${VAR} pattern as BASE_CHAIN_RPC)
• Refactor Stripe HTTP client: 30s request timeout, StripeResponse { status, body } struct preserving HTTP status codes, shared parse_stripe_response() helper (DRY)
• Add billing_keys_present: bool to health endpoint (informational only — does NOT affect 200/503 status)
• Add k6 billing correctness spec: GET /billing/stripe_config verifies publishable key is returned on deployed server
• Update deploy-phala.yml header with complete secret inventory (GitHub secrets + Phala CVM encrypted env vars)
• Add TODOS.md with P2 startup Stripe key validation (depends on client refactor shipping here)
• Log error before silently disabling billing if reqwest client fails to build

Context

Billing code exists and works locally, but is silently disabled in all deployed environments because Stripe keys were never injected. This PR closes the gap: keys flow through Phala CVM → container env → stripe::init() → billing enabled. The client refactor (HTTP status preservation + timeouts) unblocks the P2 startup validation TODO.

Test Coverage

COVERAGE: 11/12 paths tested (92%)
QUALITY:  ★★★: 9  ★★: 2  ★: 0
GAPS: 1 minor (billing_keys_present=true, blocked by private fields — trivial is_some() logic)

Pre-Landing Review

Pre-Landing Review: 2 issues (0 critical, 2 informational) — all auto-fixed.

• [AUTO-FIXED] TODOS.md:13 — stale line references → replaced with function names
• [AUTO-FIXED] TODOS.md:19 — stale branch name → replaced with PR feat: Stripe secrets pipeline — env wiring, client refactor, health check, k6 test #184
• [AUTO-FIXED] stripe.rs:41 — silent billing disable on client build failure → added error logging

Plan Completion

8/10 DONE, 2 NOT DONE (both explicitly deferred during eng review):

• Stripe webhook endpoint → future work
• Startup key validation → deferred to TODOS.md as P2

TODOS

• TODOS.md created with P2 startup Stripe key validation item

Test plan

• 7 stripe/health unit tests pass (6 parse_stripe_response + 1 billing_keys_present)
• All existing tests pass (1 pre-existing macOS failure: lit-actions socket present in dev)
• k6 billing.spec.ts validates GET /billing/stripe_config returns pk_* key on deployed server
• Manual: configure Phala CVM encrypted env vars for chipotle-next with test sandbox keys

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a planning/TODO document describing upcoming work for Stripe secret injection and a follow-up improvement to validate Stripe keys at startup, to avoid silent billing failures.

Changes:

• Introduces TODOS.md with a P2 item for startup Stripe key validation (GET /v1/balance) and associated rollout/behavior notes.

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