feat: implement per-account databases and native CouchDB attachments

- Create separate CouchDB database for each mail source (account)
- Store email attachments as native CouchDB attachments
- Add GenerateAccountDBName() for CouchDB-compatible database naming
- Update MailDocument structure to support _attachments field
- Implement StoreAttachment() for CouchDB attachment API
- Add placeholder attachment testing for every 3rd message

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

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

go/mail/imap.go

@@ -16,13 +16,21 @@ type ImapClient struct {
 
 // Message represents an email message retrieved from IMAP
 type Message struct {
-	UID     uint32
-	From    []string
-	To      []string
-	Subject string
-	Date    time.Time
-	Body    string
-	Headers map[string][]string
+	UID         uint32
+	From        []string
+	To          []string
+	Subject     string
+	Date        time.Time
+	Body        string
+	Headers     map[string][]string
+	Attachments []Attachment
+}
+
+// Attachment represents an email attachment
+type Attachment struct {
+	Filename    string
+	ContentType string
+	Content     []byte
 }
 
 // NewImapClient creates a new IMAP client from the configuration
@@ -87,6 +95,18 @@ func (c *ImapClient) GetMessages(mailbox string, since *time.Time) ([]*Message,
 			Body:    fmt.Sprintf("This is a placeholder message %d from mailbox %s", i, mailbox),
 			Headers: make(map[string][]string),
 		}
+		
+		// Add a sample attachment for testing (every 3rd message)
+		if i%3 == 0 {
+			msg.Attachments = []Attachment{
+				{
+					Filename:    fmt.Sprintf("sample_%d.txt", i),
+					ContentType: "text/plain",
+					Content:     []byte(fmt.Sprintf("Sample attachment content for message %d", i)),
+				},
+			}
+		}
+		
 		messages = append(messages, msg)
 	}