refactor: use pinned golangci-lint Docker image for linting

Refactor Dockerfile to use a separate lint stage with a pinned
golangci-lint v2.11.3 Docker image instead of installing
golangci-lint via curl in the builder stage. This follows the
pattern used by sneak/pixa.

Changes:
- Dockerfile: separate lint stage using golangci/golangci-lint:v2.11.3
  (Debian-based, pinned by sha256) with COPY --from=lint dependency
- Bump Go from 1.24 to 1.26.1 (golang:1.26.1-bookworm, pinned)
- Bump golangci-lint from v1.64.8 to v2.11.3
- Migrate .golangci.yml from v1 to v2 format (same linters, format only)
- All Docker images pinned by sha256 digest
- Fix all lint issues from the v2 linter upgrade:
  - Add package comments to all packages
  - Add doc comments to all exported types, functions, and methods
  - Fix unchecked errors (errcheck)
  - Fix unused parameters (revive)
  - Fix gosec warnings (MaxBytesReader for form parsing)
  - Fix staticcheck suggestions (fmt.Fprintf instead of WriteString)
  - Rename DeliveryTask to Task to avoid stutter (delivery.Task)
  - Rename shadowed builtin 'max' parameter
- Update README.md version requirements

internal/database/password.go

@@ -169,16 +169,16 @@ func GenerateRandomPassword(length int) (string, error) {
 	return string(password), nil
 }
 
-// cryptoRandInt generates a cryptographically secure random integer in [0, max)
-func cryptoRandInt(max int) int {
-	if max <= 0 {
-		panic("max must be positive")
+// cryptoRandInt generates a cryptographically secure random integer in [0, upperBound).
+func cryptoRandInt(upperBound int) int {
+	if upperBound <= 0 {
+		panic("upperBound must be positive")
 	}
 
 	// Calculate the maximum valid value to avoid modulo bias
-	// For example, if max=200 and we have 256 possible values,
+	// For example, if upperBound=200 and we have 256 possible values,
 	// we only accept values 0-199 (reject 200-255)
-	nBig, err := rand.Int(rand.Reader, big.NewInt(int64(max)))
+	nBig, err := rand.Int(rand.Reader, big.NewInt(int64(upperBound)))
 	if err != nil {
 		panic(fmt.Sprintf("crypto/rand error: %v", err))
 	}