feat: tiered API plans (Free/Bronze/Gold/Platinum)

- Free: $0, 30 req/min, market data only
- Bronze: $29/mo, 100 req/min, + companies & search
- Gold: $99/mo, 500 req/min, + filings & historical
- Platinum: $299/mo, 2000 req/min, all features + priority
- Plan-aware rate limiting per API key (or per IP for free)
- Usage tracking with daily aggregation
- GET /api/v1/plans — plan listing
- POST /api/v1/plans/register — instant free API key
- GET /api/v1/plans/usage — usage stats
- /pricing — dark-themed HTML pricing page
- X-RateLimit-* and X-Plan headers on every response
- Restricted endpoints return upgrade prompt
- Updated OpenAPI spec with security scheme
- 53 handlers, compiles clean

cmd/sentinela/main.go

@@ -48,6 +48,11 @@ func main() {
 		}
 	}
 
+	// Seed test API key
+	if _, err := database.CreateAPIKeyWithValue("sentinela-test-key-2026", "Test User", "test@sentinela.dev", "platinum"); err != nil {
+		slog.Warn("failed to seed test API key", "error", err)
+	}
+
 	// Start scheduler
 	syncInterval, err := time.ParseDuration(cfg.SyncInterval)
 	if err != nil {