Task 8: Standardize modeling style (TypedDict vs Pydantic)

Convert inconsistent modeling style to standardized Pydantic models for all
external-facing data structures while maintaining TypedDict compatibility where
appropriate for internal layer-private structures.

Changes:
- Converted IpLookupResult TypedDict to use IpLookupResponse Pydantic model
  in jail_service.lookup_ip() for consistency with routers
- Added GeoCacheEntry Pydantic model for geo cache repository rows
- Converted GeoCacheRow TypedDict to use GeoCacheEntry alias
- Converted ImportLogRow TypedDict to use ImportLogEntry alias
- Updated routers and services to work with Pydantic models
- Updated all tests to use Pydantic model field access (attributes)
  instead of dict subscripting

Documentation:
- Added 'Model Type Usage by Layer' section to Backend-Development.md
- Defines when TypedDict is allowed (internal structures) vs Pydantic
  (external-facing, cross-boundary data)
- Provides clear guidance on modeling conventions per layer

Benefits:
- Consistent validation and serialization behavior
- Better IDE support and type checking
- Clearer separation of concerns by layer
- Reduced maintenance cost from mixed validation approaches

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/models/geo.py

@@ -42,6 +42,33 @@ class GeoDetail(BaseModel):
     )
 
 
+class GeoCacheEntry(BaseModel):
+    """A single cached geolocation entry for an IP address.
+
+    Represents a row from the ``geo_cache`` table in the application database.
+    """
+
+    model_config = ConfigDict(strict=True)
+
+    ip: str = Field(..., description="IP address (IPv4 or IPv6).")
+    country_code: str | None = Field(
+        default=None,
+        description="ISO 3166-1 alpha-2 country code.",
+    )
+    country_name: str | None = Field(
+        default=None,
+        description="Human-readable country name.",
+    )
+    asn: str | None = Field(
+        default=None,
+        description="Autonomous System Number (e.g. ``'AS3320'``).",
+    )
+    org: str | None = Field(
+        default=None,
+        description="Organisation associated with the ASN.",
+    )
+
+
 class GeoCacheStatsResponse(BaseModel):
     """Response for ``GET /api/geo/stats``.