build: update golangci-lint to v2.12.2 with canonical config

Add the canonical .golangci.yml (v2 schema, all linters enabled with a
small documented disable list) and pin the Dockerfile lint stage to
golangci/golangci-lint:v2.12.2 by tag and digest, replacing the old
v1.64.8 digest-only pin.

Fix all findings surfaced by the v1 to v2 jump without changing any
exported signatures or behavior:

- add package and exported-symbol doc comments (revive)
- rename unused handler parameters to underscore (revive)
- check or explicitly discard error returns (errcheck, errchkjson)
- wrap errors with %w instead of %v (err113)
- use http.NewRequestWithContext instead of http.Post (noctx)
- replace fmt.Println with fmt.Fprintln(os.Stdout, ...) (forbidigo)
- name magic numbers as constants (mnd)
- add explicit slog.LevelDebug case (exhaustive)
- interface{} to any (modernize)
- move tests to the simplelog_test package (testpackage) and add
  t.Parallel() (paralleltest)
- move NewWebhookHandler above its methods (funcorder)
- whitespace, line-length, and blank-line fixes (wsl_v5, whitespace,
  nlreturn, lll, embeddedstructfieldcheck)
- nolint with justification for the intentional init/global design
  (gochecknoinits, gochecknoglobals) and interface-returning
  constructor (ireturn)

event.go

@@ -7,6 +7,8 @@ import (
 	"github.com/google/uuid"
 )
 
+// Event is a single structured log entry with a unique ID and
+// timestamp.
 type Event struct {
 	ID        uuid.UUID       `json:"id"`
 	Timestamp time.Time       `json:"timestamp"`
@@ -15,6 +17,8 @@ type Event struct {
 	Data      json.RawMessage `json:"data"`
 }
 
+// NewEvent returns an Event with a fresh ID and the current UTC
+// timestamp.
 func NewEvent(level, message string, data json.RawMessage) Event {
 	return Event{
 		ID:        uuid.New(),