refactor: use pinned golangci-lint Docker image for linting (#55)

Closes [issue #50](#50)

## Summary

Refactors the Dockerfile to use a separate lint stage with a pinned golangci-lint Docker image, following the pattern used by [sneak/pixa](https://git.eeqj.de/sneak/pixa). This replaces the previous approach of installing golangci-lint via curl in the builder stage.

## Changes

### Dockerfile
- **New `lint` stage** using `golangci/golangci-lint:v2.11.3` (Debian-based, pinned by sha256 digest) as a separate build stage
- **Builder stage** depends on lint via `COPY --from=lint /src/go.sum /dev/null` — build won't proceed unless linting passes
- **Go bumped** from 1.24 to 1.26.1 (`golang:1.26.1-bookworm`, pinned by sha256)
- **golangci-lint bumped** from v1.64.8 to v2.11.3
- All three Docker images (golangci-lint, golang, alpine) pinned by sha256 digest
- Debian-based golangci-lint image used (not Alpine) because mattn/go-sqlite3 CGO does not compile on musl (off64_t)

### Linter Config (.golangci.yml)
- Migrated from v1 to v2 format (`version: "2"` added)
- Removed linters no longer available in v2: `gofmt` (handled by `make fmt-check`), `gosimple` (merged into `staticcheck`), `typecheck` (always-on in v2)
- Same set of linters enabled — no rules weakened

### Code Fixes (all lint issues from v2 upgrade)
- Added package comments to all packages
- Added doc comments to all exported types, functions, and methods
- Fixed unchecked errors flagged by `errcheck` (sqlDB.Close, os.Setenv in tests, resp.Body.Close, fmt.Fprint)
- Fixed unused parameters flagged by `revive` (renamed to `_`)
- Fixed `gosec` G120 warnings: added `http.MaxBytesReader` before `r.ParseForm()` calls
- Fixed `staticcheck` QF1012: replaced `WriteString(fmt.Sprintf(...))` with `fmt.Fprintf`
- Fixed `staticcheck` QF1003: converted if/else chain to tagged switch
- Renamed `DeliveryTask` → `Task` to avoid package stutter (`delivery.Task` instead of `delivery.DeliveryTask`)
- Renamed shadowed builtin `max` parameter to `upperBound` in `cryptoRandInt`
- Used `t.Setenv` instead of `os.Setenv` in tests (auto-restores)

### README.md
- Updated version requirements: Go 1.26+, golangci-lint v2.11+
- Updated Dockerfile description in project structure

## Verification

`docker build .` passes cleanly — formatting check, linting, all tests, and build all succeed.

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #55
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/config/config.go

@@ -1,6 +1,8 @@
+// Package config loads application configuration from environment variables.
 package config
 
 import (
+	"errors"
 	"fmt"
 	"log/slog"
 	"os"
@@ -17,19 +19,29 @@ import (
 )
 
 const (
-	// EnvironmentDev represents development environment
+	// EnvironmentDev represents development environment.
 	EnvironmentDev = "dev"
-	// EnvironmentProd represents production environment
+	// EnvironmentProd represents production environment.
 	EnvironmentProd = "prod"
+
+	// defaultPort is the default HTTP listen port.
+	defaultPort = 8080
 )
 
-// nolint:revive // ConfigParams is a standard fx naming convention
+// ErrInvalidEnvironment is returned when WEBHOOKER_ENVIRONMENT
+// contains an unrecognised value.
+var ErrInvalidEnvironment = errors.New("invalid environment")
+
+//nolint:revive // ConfigParams is a standard fx naming convention.
 type ConfigParams struct {
 	fx.In
+
 	Globals *globals.Globals
 	Logger  *logger.Logger
 }
 
+// Config holds all application configuration loaded from
+// environment variables.
 type Config struct {
 	DataDir         string
 	Debug           bool
@@ -43,56 +55,67 @@ type Config struct {
 	log             *slog.Logger
 }
 
-// IsDev returns true if running in development environment
+// IsDev returns true if running in development environment.
 func (c *Config) IsDev() bool {
 	return c.Environment == EnvironmentDev
 }
 
-// IsProd returns true if running in production environment
+// IsProd returns true if running in production environment.
 func (c *Config) IsProd() bool {
 	return c.Environment == EnvironmentProd
 }
 
-// envString returns the value of the named environment variable, or
-// an empty string if not set.
+// envString returns the value of the named environment variable,
+// or an empty string if not set.
 func envString(key string) string {
 	return os.Getenv(key)
 }
 
-// envBool returns the value of the named environment variable parsed as a
-// boolean. Returns defaultValue if not set.
+// envBool returns the value of the named environment variable
+// parsed as a boolean. Returns defaultValue if not set.
 func envBool(key string, defaultValue bool) bool {
 	if v := os.Getenv(key); v != "" {
 		return strings.EqualFold(v, "true") || v == "1"
 	}
+
 	return defaultValue
 }
 
-// envInt returns the value of the named environment variable parsed as an
-// integer. Returns defaultValue if not set or unparseable.
+// envInt returns the value of the named environment variable
+// parsed as an integer. Returns defaultValue if not set or
+// unparseable.
 func envInt(key string, defaultValue int) int {
 	if v := os.Getenv(key); v != "" {
-		if i, err := strconv.Atoi(v); err == nil {
+		i, err := strconv.Atoi(v)
+		if err == nil {
 			return i
 		}
 	}
+
 	return defaultValue
 }
 
-// nolint:revive // lc parameter is required by fx even if unused
+// New creates a Config by reading environment variables.
+//
+//nolint:revive // lc parameter is required by fx even if unused.
 func New(lc fx.Lifecycle, params ConfigParams) (*Config, error) {
 	log := params.Logger.Get()
 
-	// Determine environment from WEBHOOKER_ENVIRONMENT env var, default to dev
+	// Determine environment from WEBHOOKER_ENVIRONMENT env var,
+	// default to dev
 	environment := os.Getenv("WEBHOOKER_ENVIRONMENT")
 	if environment == "" {
 		environment = EnvironmentDev
 	}
 
 	// Validate environment
-	if environment != EnvironmentDev && environment != EnvironmentProd {
-		return nil, fmt.Errorf("WEBHOOKER_ENVIRONMENT must be either '%s' or '%s', got '%s'",
-			EnvironmentDev, EnvironmentProd, environment)
+	if environment != EnvironmentDev &&
+		environment != EnvironmentProd {
+		return nil, fmt.Errorf(
+			"%w: WEBHOOKER_ENVIRONMENT must be '%s' or '%s', got '%s'",
+			ErrInvalidEnvironment,
+			EnvironmentDev, EnvironmentProd, environment,
+		)
 	}
 
 	// Load configuration values from environment variables
@@ -103,15 +126,16 @@ func New(lc fx.Lifecycle, params ConfigParams) (*Config, error) {
 		Environment:     environment,
 		MetricsUsername: envString("METRICS_USERNAME"),
 		MetricsPassword: envString("METRICS_PASSWORD"),
-		Port:            envInt("PORT", 8080),
+		Port:            envInt("PORT", defaultPort),
 		SentryDSN:       envString("SENTRY_DSN"),
 		log:             log,
 		params:          &params,
 	}
 
-	// Set default DataDir. All SQLite databases (main application DB
-	// and per-webhook event DBs) live here. The same default is used
-	// regardless of environment; override with DATA_DIR if needed.
+	// Set default DataDir. All SQLite databases (main application
+	// DB and per-webhook event DBs) live here. The same default is
+	// used regardless of environment; override with DATA_DIR if
+	// needed.
 	if s.DataDir == "" {
 		s.DataDir = "/var/lib/webhooker"
 	}
@@ -128,7 +152,8 @@ func New(lc fx.Lifecycle, params ConfigParams) (*Config, error) {
 		"maintenanceMode", s.MaintenanceMode,
 		"dataDir", s.DataDir,
 		"hasSentryDSN", s.SentryDSN != "",
-		"hasMetricsAuth", s.MetricsUsername != "" && s.MetricsPassword != "",
+		"hasMetricsAuth",
+		s.MetricsUsername != "" && s.MetricsPassword != "",
 	)
 
 	return s, nil