feat: per-plan cancellation reasons, renewal reminders, lifecycle history, regional tax

[OkeyAmy]

Summary

Implements four Stellar Wave features as four self-contained commits:

Issue	Commit	Scope
#1728	566aac1	Per-plan cancellation reasons + reporting
#1730	22ff7ab	Configurable renewal reminder intervals + notification preferences
#1729	9cacf01	Timestamped lifecycle state history timeline + export
#1732	f1f65c9	Per-region tax/VAT applied at invoice time

No dependency, lockfile or build-config changes.

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#1728 — Add per-plan cancellation reasons

• Reasons configurable — CancellationService.configurePlanReasons() registers a per-plan vocabulary (with optional requireReason), exposed as GET/PUT /cancellation/plans/:planId/reasons
• Aggregated in reports — analytics now carry byPlan and byPlanReason; GET /cancellation/analytics accepts an optional ?planId= filter
• Optional — unconfigured plans keep the default reason list, and a reason is only enforced when a plan opts in via requireReason

#1730 — Add renewal reminders at T-7 and T-1

• Intervals configurable — defaults to 30/7/1 days, editable at runtime via GET/PUT /api/v1/calendar/reminders/config (getConfig() / configure() on the scheduler)
• Respects notification preferences — per-subscription notificationsEnabled plus an injectable isReminderEnabled(userId, eventType) preference hook
• Tested — new specs cover custom intervals, runtime reconfiguration, disabled event types and the notifications kill switch
• Also fixed a pre-existing collision where two lead times sharing an event type overwrote each other (the dedup key now includes the lead time), which is what made custom "T-N" intervals actually produce one calendar event each

#1729 — Add lifecycle state history timeline

• Each transition timestamped — subscriptionStore records created/updated/price_changed/plan_changed/paused/resumed/cancelled events with timestamps and details, persisted alongside subscriptions (with migration/normalisation of stored state)
• Visible in the UI — "Lifecycle history" timeline card on the subscription detail screen
• Exportable — CSV export via the share sheet, built by pure helpers in src/utils/lifecycleHistory.ts

#1732 — Add regional tax and VAT calculation

• Rates configurable per region — TaxService.configureRegionRate() / getRegionRate() / listRegionRates() overrides, resolved ahead of the existing state/city lookup (resolveRegionTaxRate)
• Applied at invoice time — generateInvoice accepts region, resolves the rate when no explicit taxRate is given, and persists tax_region / tax_type via db/migrations/016_invoice_tax_region.sql (with a -- @down rollback section)
• Reported separately — tax is shown as its own labelled line (Tax (VAT 19.0% · DE)) on the generated PDF and in the invoice email, keeping it distinct from the subtotal/total

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Verification

• Backend tests — 5 affected suites, 105 tests, all pass: cancellationService, RenewalReminderScheduler, taxService, invoiceGeneration, invoiceController
• Frontend tests — 2 pure-TS suites, 16 tests, all pass (via the backend jest config workaround, as the RN preset is currently broken per AGENTS.md): lifecycleHistory, subscriptionLifecycleHistory
• Prettier — clean on all changed files
• ESLint — no new issues on changed frontend files vs main (the 4 errors / 5 warnings reported there are identical on main)
• TypeScript — no new errors vs main on both configs: root tsc --noEmit (616 pre-existing) and backend/tsconfig.json (215 pre-existing)
• Android build / Rust gates — not run locally (the Expo/RN dependency install could not complete in this environment; no smart-contract code touched)

Checklist

• Lint: ESLint and Prettier checked on changed files
• Type Check: no new errors introduced
• Tests: new features have tests, affected suites green
• Build: not run locally (see above)
• Rust gates: N/A — no smart-contract changes
• New code has appropriate TypeScript types
• No hardcoded secrets or credentials
• New features have corresponding tests
• Documentation: not updated (no docs cover these domains yet)

Notes

• cancellationRouter and invoiceController are still not mounted in createApiServer.ts (pre-existing); the new routes/behaviour follow the existing conventions of those modules.
• The Add renewal reminders at T-7 and T-1 #1730 config endpoints are mounted at /api/v1/calendar/reminders/config.

Closes #1728
Closes #1730
Closes #1729
Closes #1732

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