chore(dev): pass STRIPE_* env to backend container; add repo-root .env.example

The backend container had no Stripe env vars wired through compose, so
sync_stripe_plan_ids.py and any in-app Stripe calls would short-circuit
even when sk_test_ was set in the repo .env. Adds STRIPE_SECRET_KEY,
STRIPE_PUBLISHABLE_KEY, STRIPE_WEBHOOK_SECRET pass-throughs.

Also flips REQUIRE_INVITE_CODE to false in the dev compose (matches the
working state on this machine — Phase 2 self-serve has been gating that
behavior on SELF_SERVE_ENABLED + the upcoming INTERNAL_TESTER_EMAILS
allowlist anyway).

Adds a repo-root .env.example documenting the variables compose itself
reads (REPO_ROOT, POSTGRES_PORT, secrets) — separate from
backend/.env.example which documents the backend service env.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.env.example

@@ -0,0 +1,10 @@
+REPO_ROOT=/opt/docker/code-server/workspace/resolutionflow
+POSTGRES_PORT=5433
+SECRET_KEY=
+ANTHROPIC_API_KEY=
+GOOGLE_AI_API_KEY=
+
+STRIPE_SECRET_KEY=sk_test_
+STRIPE_PUBLISHABLE_KEY=pk_test_
+STRIPE_WEBHOOK_SECRET=whsec_
+VITE_STRIPE_PUBLISHABLE_KEY=pk_test_