feat: contact form backend for linux and freebsd servers

Assisted-by: GLM 5.3 Flash

internal/storage/submissions.go

@@ -0,0 +1,85 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: MIT
+
+//go:build linux || freebsd
+
+package storage
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sync"
+	"time"
+)
+
+// Submission is one archived form submission, one JSON object per line.
+// Newsletter forms do not write here: they persist through the double
+// opt-in subscriber log instead.
+type Submission struct {
+	Form      string    `json:"form"`
+	Name      string    `json:"name,omitempty"`
+	Email     string    `json:"email"`
+	Service   string    `json:"service,omitempty"`
+	Message   string    `json:"message"`
+	IP        string    `json:"ip,omitempty"`
+	CreatedAt time.Time `json:"created_at"`
+}
+
+// ArchiveStore is a file-based append-only log of submissions. All methods
+// are safe for concurrent use. The file and its parent directory are
+// created lazily on first Append, so a form that archives nothing writes
+// nothing.
+type ArchiveStore struct {
+	path string
+	mu   sync.Mutex
+}
+
+// NewArchiveStore returns a store that appends to path.
+func NewArchiveStore(path string) *ArchiveStore {
+	return &ArchiveStore{path: path}
+}
+
+// Path returns the file path this store writes to.
+func (s *ArchiveStore) Path() string {
+	return s.path
+}
+
+// Append writes sub as a single JSON line to the log.
+func (s *ArchiveStore) Append(sub Submission) error {
+	if sub.CreatedAt.IsZero() {
+		sub.CreatedAt = time.Now().UTC()
+	}
+	line, err := json.Marshal(sub)
+	if err != nil {
+		return fmt.Errorf("marshal submission: %w", err)
+	}
+	line = append(line, '\n')
+
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return appendLine(s.path, line)
+}
+
+// appendLine writes one marshalled JSON line to an append-only log,
+// creating the file and its parent directory on first use. The caller
+// holds the store's lock.
+func appendLine(path string, line []byte) (err error) {
+	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
+		return fmt.Errorf("mkdir %s: %w", filepath.Dir(path), err)
+	}
+	f, err := os.OpenFile(path, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
+	if err != nil {
+		return fmt.Errorf("open %s: %w", path, err)
+	}
+	defer func() {
+		if cerr := f.Close(); cerr != nil && err == nil {
+			err = fmt.Errorf("close %s: %w", path, cerr)
+		}
+	}()
+	if _, err := f.Write(line); err != nil {
+		return fmt.Errorf("write %s: %w", path, err)
+	}
+	return nil
+}