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/database.go

@@ -1,3 +1,4 @@
+// Package database provides SQLite persistence for webhooks, events, and users.
 package database
 
 import (
@@ -19,19 +20,21 @@ import (
 	"sneak.berlin/go/webhooker/internal/logger"
 )
 
-// nolint:revive // DatabaseParams is a standard fx naming convention
+//nolint:revive // DatabaseParams is a standard fx naming convention.
 type DatabaseParams struct {
 	fx.In
 	Config *config.Config
 	Logger *logger.Logger
 }
 
+// Database manages the main SQLite connection and schema migrations.
 type Database struct {
 	db     *gorm.DB
 	log    *slog.Logger
 	params *DatabaseParams
 }
 
+// New creates a Database that connects on fx start and disconnects on stop.
 func New(lc fx.Lifecycle, params DatabaseParams) (*Database, error) {
 	d := &Database{
 		params: &params,
@@ -149,6 +152,7 @@ func (d *Database) close() error {
 	return nil
 }
 
+// DB returns the underlying GORM database handle.
 func (d *Database) DB() *gorm.DB {
 	return d.db
 }