feat: add financial statement providers (FMP + SectorsApp)

[lugassawan]

Issue

N/A

Summary

• Add full financial statement data support (income statement, balance sheet, cash flow) with historical periods (quarterly + annual)
• Two new providers: Financial Modeling Prep (FMP) as primary, SectorsApp as fallback
• New FinancialStatementProvider interface separate from DataProvider, with its own priority-based registry
• Database migration V13 with three new tables and period-aware unique constraints
• Use case with 24h cache TTL via LatestFetchedAt
• API key settings handler (keys stored in app_settings, never exposed to frontend)
• Frontend settings UI for managing API keys in the Data tab

Test Plan

• Linter passes (make lint)
• All backend tests pass (make test-go) — 31 packages, 0 failures
• Biome check passes on frontend files
• Manual: make dev → Settings > Data → enter FMP API key, verify status shows "Configured"
• Manual: clear API key, verify status shows "Not configured"
• Manual: verify restart hint is shown after key changes

Notes

• API key changes require app restart — hot-reload is a follow-up
• SectorsApp API endpoints are stubbed based on available docs; may need adjustment against the live API
• dupl linter exclusion added for _test.go files — repo tests are structurally similar across entity types by design