Add config package: TOML loading with env overrides

- Base type with standard sections (Server, Database, MCIAS, Log)
- Duration wrapper type for TOML string→time.Duration decoding
- Generic Load[T] with TOML parse, reflection-based env overrides,
  defaults, required field validation, optional Validator interface
- Env overrides: PREFIX_SECTION_FIELD for string, duration, bool,
  []string (comma-separated)
- WebConfig exported for services with web UIs (not embedded in Base)
- 16 tests covering full/minimal configs, defaults, env overrides,
  validation, error cases

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

config/duration.go

@@ -0,0 +1,28 @@
+package config
+
+import (
+	"fmt"
+	"time"
+)
+
+// Duration is a time.Duration that can be unmarshalled from a TOML string
+// (e.g., "30s", "5m"). go-toml v2 does not natively decode strings into
+// time.Duration, so this wrapper implements encoding.TextUnmarshaler.
+type Duration struct {
+	time.Duration
+}
+
+// UnmarshalText implements encoding.TextUnmarshaler for TOML string decoding.
+func (d *Duration) UnmarshalText(text []byte) error {
+	parsed, err := time.ParseDuration(string(text))
+	if err != nil {
+		return fmt.Errorf("invalid duration %q: %w", string(text), err)
+	}
+	d.Duration = parsed
+	return nil
+}
+
+// MarshalText implements encoding.TextMarshaler for TOML string encoding.
+func (d Duration) MarshalText() ([]byte, error) {
+	return []byte(d.String()), nil
+}