refactor(models): split models.py into topical submodules

decnet/web/db/models.py was approaching 1000 lines across User/Log/
Attacker/Swarm/Topology/Workers/Updater/Health domains. Split into a
package with one module per domain; __init__.py re-exports every symbol
so all 52 call sites keep importing from decnet.web.db.models
unchanged.

decnet/web/db/models/_base.py

@@ -0,0 +1,23 @@
+"""Shared column/validator helpers used across model domain modules."""
+from datetime import datetime
+from typing import Annotated, Any, Optional
+
+from pydantic import BeforeValidator
+from sqlalchemy import Text
+from sqlalchemy.dialects.mysql import MEDIUMTEXT
+
+# Use on columns that accumulate over an attacker's lifetime (commands,
+# fingerprints, state blobs).  TEXT on MySQL caps at 64 KiB; MEDIUMTEXT
+# stretches to 16 MiB.  SQLite has no fixed-width text types so Text()
+# stays unchanged there.
+_BIG_TEXT = Text().with_variant(MEDIUMTEXT(), "mysql")
+
+
+def _normalize_null(v: Any) -> Any:
+    if isinstance(v, str) and v.lower() in ("null", "undefined", ""):
+        return None
+    return v
+
+
+NullableDatetime = Annotated[Optional[datetime], BeforeValidator(_normalize_null)]
+NullableString = Annotated[Optional[str], BeforeValidator(_normalize_null)]