Complete Phase 3, Phase 5 polish and hardening

Phase 3 — Investments:
- Multi-currency support: holdings track purchase currency, FX rates convert to base for totals
- Capital gains report using UK Section 104 pool method, grouped by tax year
- Capital Gains tab added to Reports page

Phase 5 — Polish & Hardening:
- Mobile-responsive layout: bottom nav, sidebar hidden on mobile, logo in TopBar, compact header buttons, hover-only actions now always visible on touch
- Backup system: encrypted GPG backups via backup.sh, nightly scheduler job, admin API (list/trigger/download/restore), Settings UI with drag-to-restore confirmation
- Docker entrypoint with gosu privilege drop to fix bind-mount ownership on fresh deployments
- OWASP fixes: refresh token now bound to its session (new refresh_token_hash column + migration), CSRF secure flag tied to environment, IP-level rate limiting on login, TOTPEnableRequest Pydantic schema replaces raw dict
- AES-256-GCM key rotation script (rotate_keys.py) with dry-run mode and atomic DB transaction
- CLAUDE.md added for AI-assisted development context
- README updated: correct reverse proxy port, accurate backup/restore commands, key rotation instructions

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/schemas/report.py

@@ -96,6 +96,20 @@ class SpendingTrendsReport(BaseModel):
     currency: str
 
 
+class SavingsRatePoint(BaseModel):
+    month: str
+    income: Decimal
+    expenses: Decimal
+    savings: Decimal
+    savings_rate: Decimal
+
+
+class SavingsRateReport(BaseModel):
+    points: list[SavingsRatePoint]
+    avg_savings_rate: Decimal
+    currency: str
+
+
 class BalanceSheetAccount(BaseModel):
     id: str
     name: str