feat: implement Go-based mail2couch with working IMAP and CouchDB integration

- Add configuration system with automatic file discovery (current dir, config subdir, user home, XDG config)
- Implement IMAP client with TLS connection, authentication, and mailbox listing
- Add CouchDB integration with database creation and document storage
- Support folder filtering (include/exclude) and date filtering (since parameter)
- Include duplicate detection to prevent re-storing existing messages
- Add comprehensive error handling and logging throughout
- Structure code in clean packages: config, mail, couch
- Application currently uses placeholder messages to test the storage pipeline
- Ready for real IMAP message parsing implementation

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

go/couch/couch.go

@@ -0,0 +1,130 @@
+package couch
+
+import (
+	"context"
+	"fmt"
+	"net/url"
+	"time"
+
+	"github.com/go-kivik/kivik/v4"
+	_ "github.com/go-kivik/kivik/v4/couchdb" // The CouchDB driver
+	"mail2couch/config"
+	"mail2couch/mail"
+)
+
+// Client wraps the Kivik client
+type Client struct {
+	*kivik.Client
+}
+
+// MailDocument represents an email message stored in CouchDB
+type MailDocument struct {
+	ID        string              `json:"_id,omitempty"`
+	Rev       string              `json:"_rev,omitempty"`
+	SourceUID string              `json:"sourceUid"` // Unique ID from the mail source (e.g., IMAP UID)
+	Mailbox   string              `json:"mailbox"`   // Source mailbox name
+	From      []string            `json:"from"`
+	To        []string            `json:"to"`
+	Subject   string              `json:"subject"`
+	Date      time.Time           `json:"date"`
+	Body      string              `json:"body"`
+	Headers   map[string][]string `json:"headers"`
+	StoredAt  time.Time           `json:"storedAt"` // When the document was stored
+	DocType   string              `json:"docType"`  // Always "mail"
+}
+
+// NewClient creates a new CouchDB client from the configuration
+func NewClient(cfg *config.CouchDbConfig) (*Client, error) {
+	parsedURL, err := url.Parse(cfg.URL)
+	if err != nil {
+		return nil, fmt.Errorf("invalid couchdb url: %w", err)
+	}
+
+	parsedURL.User = url.UserPassword(cfg.User, cfg.Password)
+	dsn := parsedURL.String()
+
+	client, err := kivik.New("couch", dsn)
+	if err != nil {
+		return nil, err
+	}
+
+	return &Client{client}, nil
+}
+
+// EnsureDB ensures that the configured database exists.
+func (c *Client) EnsureDB(ctx context.Context, dbName string) error {
+	exists, err := c.DBExists(ctx, dbName)
+	if err != nil {
+		return err
+	}
+	if !exists {
+		return c.CreateDB(ctx, dbName)
+	}
+	return nil
+}
+
+// ConvertMessage converts an IMAP message to a MailDocument
+func ConvertMessage(msg *mail.Message, mailbox string) *MailDocument {
+	docID := fmt.Sprintf("%s_%d", mailbox, msg.UID)
+
+	return &MailDocument{
+		ID:        docID,
+		SourceUID: fmt.Sprintf("%d", msg.UID),
+		Mailbox:   mailbox,
+		From:      msg.From,
+		To:        msg.To,
+		Subject:   msg.Subject,
+		Date:      msg.Date,
+		Body:      msg.Body,
+		Headers:   msg.Headers,
+		StoredAt:  time.Now(),
+		DocType:   "mail",
+	}
+}
+
+// StoreMessage stores a mail message in CouchDB
+func (c *Client) StoreMessage(ctx context.Context, dbName string, doc *MailDocument) error {
+	db := c.DB(dbName)
+	if db.Err() != nil {
+		return db.Err()
+	}
+
+	// Check if document already exists
+	exists, err := c.DocumentExists(ctx, dbName, doc.ID)
+	if err != nil {
+		return fmt.Errorf("failed to check if document exists: %w", err)
+	}
+
+	if exists {
+		return nil // Document already exists, skip
+	}
+
+	// Store the document
+	_, err = db.Put(ctx, doc.ID, doc)
+	if err != nil {
+		return fmt.Errorf("failed to store document: %w", err)
+	}
+
+	return nil
+}
+
+// StoreMessages stores multiple mail messages in CouchDB
+func (c *Client) StoreMessages(ctx context.Context, dbName string, docs []*MailDocument) error {
+	for _, doc := range docs {
+		if err := c.StoreMessage(ctx, dbName, doc); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// DocumentExists checks if a document with the given ID already exists.
+func (c *Client) DocumentExists(ctx context.Context, dbName, docID string) (bool, error) {
+	db := c.DB(dbName)
+	if db.Err() != nil {
+		return false, db.Err()
+	}
+
+	row := db.Get(ctx, docID)
+	return row.Err() == nil, nil
+}