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

Replace .golangci.yml with the canonical v2-schema config
(default: all minus six disabled linters, lll 88, tests included)
and bump every golangci-lint pin to v2.12.2:

- Dockerfile: golangci/golangci-lint:v2.12.2-alpine (hash-pinned)
- script/bootstrap: GOLANGCI_LINT_VERSION 2.12.2 with new
  linux-amd64/arm64 release-archive sha256 pins

Fix all 747 findings the stricter config surfaces, with no behavior
changes: t.Parallel() throughout the test suite, static sentinel
errors and errors.Is comparisons, checked error returns, context
propagation (contextcheck/noctx), 88-column wrapping, extracted
constants and helpers for goconst/dupl/funlen/cyclop, exhaustive
switch cases replicating existing defaults, and white-box test files
renamed to *_internal_test.go for testpackage. Three
nolint:tagliatelle directives preserve the existing snake_case JSON
wire and on-disk metadata formats.

internal/handlers/imageenc.go

@@ -15,8 +15,9 @@ import (
 	"sneak.berlin/go/pixa/internal/imgcache"
 )
 
-// HandleImageEnc handles requests to /v1/e/{token}/* for encrypted image URLs.
-// The trailing path (e.g., /img.jpg) is ignored but helps browsers identify the content type.
+// HandleImageEnc handles requests to /v1/e/{token}/* for encrypted
+// image URLs. The trailing path (e.g., /img.jpg) is ignored but helps
+// browsers identify the content type.
 func (s *Handlers) HandleImageEnc() http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		ctx := r.Context()
@@ -57,7 +58,8 @@ func (s *Handlers) HandleImageEnc() http.HandlerFunc {
 			"format", req.Format,
 		)
 
-		// Fetch and process the image (no signature validation needed - encrypted URL is trusted)
+		// Fetch and process the image (no signature validation
+		// needed - encrypted URL is trusted)
 		resp, err := s.imgSvc.Get(ctx, req)
 		if err != nil {
 			s.handleImageError(w, err)
@@ -68,6 +70,7 @@ func (s *Handlers) HandleImageEnc() http.HandlerFunc {
 
 		// Set response headers
 		w.Header().Set("Content-Type", resp.ContentType)
+
 		if resp.ContentLength > 0 {
 			w.Header().Set("Content-Length", strconv.FormatInt(resp.ContentLength, 10))
 		}