Extract magic numbers to named constants

[NP-compete]

Summary

Magic numbers are scattered throughout the codebase. These should be extracted to named constants for clarity and maintainability.

Current State

```go
// internal/api/router.go:42
r.Use(chimiddleware.Timeout(60 * time.Second))

// cmd/server/main.go:114-116
srv := &http.Server{
ReadTimeout: 30 * time.Second,
WriteTimeout: 30 * time.Second,
IdleTimeout: 120 * time.Second,
}

// cmd/server/main.go:146
ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)

// internal/tools/long_operation_sdk.go:52-57
if input.Seconds < 1 {
input.Seconds = 1
}
if input.Seconds > 60 {
input.Seconds = 60
}

// test/integration_test.go:126
if pag["maxPageSize"] != float64(100) {
```

Expected Outcome

Create constants file and use throughout:

```go
// internal/constants/constants.go
package constants

import "time"

const (
// HTTP Server Timeouts
DefaultRequestTimeout = 60 * time.Second
DefaultReadTimeout = 30 * time.Second
DefaultWriteTimeout = 30 * time.Second
DefaultIdleTimeout = 120 * time.Second
DefaultShutdownTimeout = 30 * time.Second

// Pagination
DefaultPageSize = 10
MaxPageSize     = 100

// Long Operation Tool
MinOperationSeconds = 1
MaxOperationSeconds = 60

// Rate Limiting (for future use)
DefaultRateLimit = 100
DefaultBurstSize = 20

)
```

Then use:

```go
import "github.com/NP-compete/gomcp/internal/constants"

r.Use(chimiddleware.Timeout(constants.DefaultRequestTimeout))
```

Files to Update

1. `internal/api/router.go`
2. `cmd/server/main.go`
3. `internal/tools/long_operation_sdk.go`
4. `internal/pagination/pagination.go`
5. `test/integration_test.go`

Acceptance Criteria

• Constants file created
• All magic numbers replaced with named constants
• Constants have descriptive names
• Constants have comments explaining their purpose
• All tests pass