docs: add France legal/tax notes to Stripe plan

Business structure (auto-entrepreneur recommended for solo launch),
2026 VAT thresholds for services (37,500€ / 41,250€), Stripe France
fee rates, Stripe Tax as an optional add-on, and an explicit open
question on cross-border EU VAT (OSS) that needs accountant
confirmation before billing non-French EU customers. Flagged as a
starting point for an accountant conversation, not a substitute.

STRIPE_PLAN.md

@@ -238,3 +238,24 @@ Suggested build order (each step shippable/testable on its own, matching this se
 6. Family groups: `familyGroups`/`familyMembers` schema, invite/join/remove endpoints, tier-resolution change in `lib/tiers.ts`, the family panel in Settings (§1a, §7.5) — ship after Pro is proven working end-to-end, since it's the most novel piece and easiest to get subtly wrong (tier-resolution fallback logic especially).
 7. Admin insights dashboard (§6.3) — pure read-side, can land anytime after step 3.
 8. Switch test-mode keys → live-mode keys as the very last step, after a full test-mode Checkout→webhook→tier-change dry run (Pro solo *and* Family group).
+
+---
+
+## 10. France legal/tax notes
+
+**Not legal or tax advice** — this is a starting point for a conversation
+with an actual French accountant (expert-comptable) before real money
+moves through live-mode keys, not a substitute for one. Confirm the
+specifics for your situation before launch; figures below are current as
+of mid-2026 and can change.
+
+- **Business structure**: for a solo launch, **auto-entrepreneur** (micro-entreprise) is the cheapest legal structure — no share capital, simplified accounting, charges (URSSAF) only apply to revenue actually invoiced, nothing owed at €0 revenue. Only worth moving to SASU/EURL if you want stricter personal/business asset separation, outside investment, or you exceed the auto-entrepreneur revenue ceiling.
+- **VAT (TVA) thresholds, 2026, SaaS/subscriptions classified as "prestation de services"**:
+  - Franchise en base: **€37,500** revenue — below this, no VAT charged on French customers.
+  - Tolerance ceiling: **€41,250** — crossing this mid-year means VAT applies immediately from the 1st of that month; between the two thresholds you get until the following January 1st.
+  - Unchanged for 2026 (a threshold-unification reform was dropped by Parliament in late 2025).
+- **Open question — cross-border EU customers**: the French franchise en base only covers domestic (France) sales. Selling subscriptions to customers in *other* EU countries falls under the EU's digital-services VAT regime (OSS — One-Stop-Shop), which has its own rules/threshold, separate from the French one above. **Needs accountant confirmation before enabling billing for non-French EU customers** — not resolved by this plan, flagging rather than guessing at numbers.
+- **Stripe fees, France-domestic**: 1.5% + €0.25 per transaction (EEA cards). EEA corporate/commercial cards: 1.9% + €0.25. Non-EEA cards: +3.25% on top (~4.75% + €0.25 total). Card issued outside your Stripe account's country: +1.5%. Currency mismatch: +1–2%.
+- **Stripe Tax** (optional automated VAT calculation/filing): 0.5% per transaction in jurisdictions where you're registered to collect tax, plus a small per-calculation API fee above the included quota. Worth it once billing spans multiple VAT jurisdictions and manual rate-tracking gets error-prone; skip it while VAT-exempt under the franchise en base.
+
+Practical order: stay auto-entrepreneur + franchise en base (no VAT complexity) for the initial France-only launch, get the OSS/cross-border question answered before opening Checkout to other EU countries, and only evaluate Stripe Tax once that becomes relevant.