feat(mail): add governed POP3 legacy import

src/govoplan_mail/backend/config.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+from pydantic import Field, model_validator
+
 from govoplan_core.mail.config import (
     ImapConfig,
     ImapFolderMappings,
@@ -12,10 +14,49 @@ from govoplan_core.mail.config import (
     normalize_split_transport_credentials,
 )
 
+
+class Pop3ServerConfig(StrictModel):
+    """Server-only settings for an explicitly enabled legacy POP3 source."""
+
+    host: str | None = None
+    port: int | None = Field(default=None, ge=1, le=65535)
+    security: TransportSecurity = TransportSecurity.TLS
+    timeout_seconds: int = Field(default=30, ge=1, le=300)
+    max_message_bytes: int = Field(default=25 * 1024 * 1024, ge=1_024, le=50 * 1024 * 1024)
+    max_batch_bytes: int = Field(default=100 * 1024 * 1024, ge=1_048_576, le=500 * 1024 * 1024)
+    preview_body_lines: int = Field(default=20, ge=0, le=100)
+    legacy_import_enabled: bool = False
+    allow_delete_after_import: bool = False
+
+    @model_validator(mode="after")
+    def apply_default_port(self) -> "Pop3ServerConfig":
+        if self.port is None:
+            self.port = 995 if self.security == TransportSecurity.TLS else 110
+        if self.legacy_import_enabled and not str(self.host or "").strip():
+            raise ValueError(
+                "POP3 host is required when legacy import is enabled"
+            )
+        if self.max_batch_bytes < self.max_message_bytes:
+            raise ValueError(
+                "POP3 batch size limit cannot be lower than the per-message limit"
+            )
+        if self.allow_delete_after_import and not self.legacy_import_enabled:
+            raise ValueError(
+                "POP3 delete-after-import cannot be enabled while legacy import is disabled"
+            )
+        return self
+
+
+class Pop3Config(Pop3ServerConfig):
+    username: str | None = None
+    password: str | None = None
+
 __all__ = [
     "ImapConfig",
     "ImapFolderMappings",
     "ImapServerConfig",
+    "Pop3Config",
+    "Pop3ServerConfig",
     "SmtpConfig",
     "SmtpServerConfig",
     "StrictModel",