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)

tools/relp_log_trial.go

@@ -1,3 +1,5 @@
+// Command relp_log_trial emits sample log messages through simplelog's
+// default slog handler for manual testing.
 package main
 
 import (
@@ -6,11 +8,22 @@ import (
 	_ "sneak.berlin/go/simplelog" // Using underscore to only invoke init()
 )
 
+// examplePort is the sample database port logged below.
+const examplePort = 5432
+
 func main() {
 	// Send some test messages with structured data
 	slog.Info("Starting the application", slog.String("status", "initialized"))
-	slog.Info("Attempting to connect to database", slog.String("host", "localhost"), slog.Int("port", 5432))
+	slog.Info(
+		"Attempting to connect to database",
+		slog.String("host", "localhost"),
+		slog.Int("port", examplePort),
+	)
 	slog.Warn("Using default configuration", slog.String("configuration", "default"))
-	slog.Error("Failed to load module", slog.String("module", "finance"), slog.String("error", "module not found"))
+	slog.Error(
+		"Failed to load module",
+		slog.String("module", "finance"),
+		slog.String("error", "module not found"),
+	)
 	slog.Info("Shutting down the application", slog.String("status", "stopped"))
 }