feat: refactor IMAP config to DB, add multi-account mail sources admin UI

Agent-Logs-Url: https://github.com/christianlouis/dmarq/sessions/f141cd3b-8d0a-4c7e-b479-ee973c209c11

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

backend/app/models/mail_source.py

@@ -0,0 +1,53 @@
+from datetime import datetime
+
+from sqlalchemy import Boolean, Column, DateTime, Integer, String, Text
+
+from app.core.database import Base
+
+
+class MailSource(Base):
+    """
+    Mail source configuration model.
+
+    Stores credentials and settings for a mail inbox used to retrieve DMARC
+    aggregate reports.  The ``method`` field determines how the connection is
+    made and which additional fields are relevant:
+
+    - ``IMAP``       – standard IMAP4 (over SSL/TLS or STARTTLS)
+    - ``POP3``       – POP3 inbox (stub for future implementation)
+    - ``GMAIL_API``  – Gmail API with OAuth 2.0 (stub for future implementation)
+    """
+
+    __tablename__ = "mail_sources"
+
+    id = Column(Integer, primary_key=True, index=True)
+
+    # Human-readable label for the source
+    name = Column(String, nullable=False)
+
+    # Connection method – determines which fields are used at runtime
+    method = Column(String, nullable=False, default="IMAP")  # IMAP | POP3 | GMAIL_API
+
+    # Connection details (used by IMAP and POP3)
+    server = Column(String, nullable=True)
+    port = Column(Integer, nullable=True, default=993)
+    username = Column(String, nullable=True)
+    # NOTE: password is stored in plaintext.  In a production environment this
+    # field should be encrypted at the application layer before persisting.
+    password = Column(Text, nullable=True)
+    use_ssl = Column(Boolean, default=True)
+    folder = Column(String, default="INBOX")
+
+    # Polling behaviour
+    polling_interval = Column(Integer, default=60)  # minutes
+
+    # Source lifecycle
+    enabled = Column(Boolean, default=True, index=True)
+    last_checked = Column(DateTime, nullable=True)
+
+    # Timestamps
+    created_at = Column(DateTime, default=datetime.utcnow)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+
+    def __repr__(self):
+        return f"<MailSource id={self.id} name={self.name!r} method={self.method!r}>"