(feature) Add host config editing view

Add a staff-only Django form for editing operational host settings while keeping
host identity stable. Support address, enablement, SSH/source overrides,
include/exclude lists, rsync extra args, and retention settings using the same
SQL-backed HostConfig model consumed by backup and scheduler flows.

Parse newline-separated list fields into JSON lists, preserve nullable
excludes_replace semantics, and cover rendering plus update behavior with view
tests.

src/pobsync_backend/forms.py

@@ -2,10 +2,68 @@ from __future__ import annotations
 
 from django import forms
 
-from .models import ScheduleConfig
+from .models import HostConfig, ScheduleConfig
 from .scheduler import parse_cron_expr
 
 
+class NewlineListField(forms.CharField):
+    widget = forms.Textarea
+
+    def __init__(self, *args, **kwargs) -> None:
+        kwargs.setdefault("required", False)
+        super().__init__(*args, **kwargs)
+
+    def prepare_value(self, value):
+        if isinstance(value, list):
+            return "\n".join(str(item) for item in value)
+        return value
+
+    def to_python(self, value) -> list[str]:
+        if not value:
+            return []
+        if isinstance(value, list):
+            return [str(item).strip() for item in value if str(item).strip()]
+        return [line.strip() for line in str(value).splitlines() if line.strip()]
+
+
+class NullableNewlineListField(NewlineListField):
+    def to_python(self, value) -> list[str] | None:
+        parsed = super().to_python(value)
+        return parsed or None
+
+
+class HostConfigForm(forms.ModelForm):
+    includes = NewlineListField(help_text="One include path per line. Leave empty to include defaults.")
+    excludes_add = NewlineListField(help_text="One additional exclude pattern per line.")
+    excludes_replace = NullableNewlineListField(
+        help_text="Optional. When set, replaces global excludes; one pattern per line."
+    )
+    rsync_extra_args = NewlineListField(help_text="One extra rsync argument per line.")
+
+    class Meta:
+        model = HostConfig
+        fields = (
+            "address",
+            "enabled",
+            "ssh_user",
+            "ssh_port",
+            "source_root",
+            "includes",
+            "excludes_add",
+            "excludes_replace",
+            "rsync_extra_args",
+            "retention_daily",
+            "retention_weekly",
+            "retention_monthly",
+            "retention_yearly",
+        )
+        help_texts = {
+            "ssh_user": "Leave empty to use the global SSH user.",
+            "ssh_port": "Leave empty to use the global SSH port.",
+            "source_root": "Leave empty to use the global default source root.",
+        }
+
+
 class ScheduleConfigForm(forms.ModelForm):
     cron_expr = forms.CharField(
         label="Cron expression",