refactor(core)!: Add context, config, and structured logging

Introduces `context.Context` to the `FetchCourse` method and its call chain, allowing for cancellable network requests and timeouts. This improves application robustness when fetching remote course data.

A new configuration package centralizes application settings, loading them from environment variables with sensible defaults for base URL, request timeout, and logging.

Standard `log` and `fmt` calls are replaced with a structured logging system built on `slog`, supporting both JSON and human-readable text formats.

This change also includes:
- Extensive benchmarks and example tests.
- Simplified Go doc comments across several packages.

BREAKING CHANGE: The `NewArticulateParser` constructor signature has been updated to accept a logger, base URL, and timeout, which are now supplied via the new configuration system.

internal/config/config_test.go

@@ -0,0 +1,118 @@
+// Package config_test provides tests for the config package.
+package config
+
+import (
+	"log/slog"
+	"os"
+	"testing"
+	"time"
+)
+
+func TestLoad(t *testing.T) {
+	// Clear environment
+	os.Clearenv()
+
+	cfg := Load()
+
+	if cfg.BaseURL != DefaultBaseURL {
+		t.Errorf("Expected BaseURL '%s', got '%s'", DefaultBaseURL, cfg.BaseURL)
+	}
+
+	if cfg.RequestTimeout != DefaultRequestTimeout {
+		t.Errorf("Expected timeout %v, got %v", DefaultRequestTimeout, cfg.RequestTimeout)
+	}
+
+	if cfg.LogLevel != DefaultLogLevel {
+		t.Errorf("Expected log level %v, got %v", DefaultLogLevel, cfg.LogLevel)
+	}
+
+	if cfg.LogFormat != DefaultLogFormat {
+		t.Errorf("Expected log format '%s', got '%s'", DefaultLogFormat, cfg.LogFormat)
+	}
+}
+
+func TestLoad_WithEnvironmentVariables(t *testing.T) {
+	// Set environment variables
+	os.Setenv("ARTICULATE_BASE_URL", "https://test.example.com")
+	os.Setenv("ARTICULATE_REQUEST_TIMEOUT", "60")
+	os.Setenv("LOG_LEVEL", "debug")
+	os.Setenv("LOG_FORMAT", "json")
+	defer os.Clearenv()
+
+	cfg := Load()
+
+	if cfg.BaseURL != "https://test.example.com" {
+		t.Errorf("Expected BaseURL 'https://test.example.com', got '%s'", cfg.BaseURL)
+	}
+
+	if cfg.RequestTimeout != 60*time.Second {
+		t.Errorf("Expected timeout 60s, got %v", cfg.RequestTimeout)
+	}
+
+	if cfg.LogLevel != slog.LevelDebug {
+		t.Errorf("Expected log level Debug, got %v", cfg.LogLevel)
+	}
+
+	if cfg.LogFormat != "json" {
+		t.Errorf("Expected log format 'json', got '%s'", cfg.LogFormat)
+	}
+}
+
+func TestGetLogLevelEnv(t *testing.T) {
+	tests := []struct {
+		name     string
+		value    string
+		expected slog.Level
+	}{
+		{"debug lowercase", "debug", slog.LevelDebug},
+		{"debug uppercase", "DEBUG", slog.LevelDebug},
+		{"info lowercase", "info", slog.LevelInfo},
+		{"info uppercase", "INFO", slog.LevelInfo},
+		{"warn lowercase", "warn", slog.LevelWarn},
+		{"warn uppercase", "WARN", slog.LevelWarn},
+		{"warning lowercase", "warning", slog.LevelWarn},
+		{"error lowercase", "error", slog.LevelError},
+		{"error uppercase", "ERROR", slog.LevelError},
+		{"invalid value", "invalid", slog.LevelInfo},
+		{"empty value", "", slog.LevelInfo},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			os.Clearenv()
+			if tt.value != "" {
+				os.Setenv("TEST_LOG_LEVEL", tt.value)
+			}
+			result := getLogLevelEnv("TEST_LOG_LEVEL", slog.LevelInfo)
+			if result != tt.expected {
+				t.Errorf("Expected %v, got %v", tt.expected, result)
+			}
+		})
+	}
+}
+
+func TestGetDurationEnv(t *testing.T) {
+	tests := []struct {
+		name     string
+		value    string
+		expected time.Duration
+	}{
+		{"valid duration", "45", 45 * time.Second},
+		{"zero duration", "0", 0},
+		{"invalid duration", "invalid", 30 * time.Second},
+		{"empty value", "", 30 * time.Second},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			os.Clearenv()
+			if tt.value != "" {
+				os.Setenv("TEST_DURATION", tt.value)
+			}
+			result := getDurationEnv("TEST_DURATION", 30*time.Second)
+			if result != tt.expected {
+				t.Errorf("Expected %v, got %v", tt.expected, result)
+			}
+		})
+	}
+}