(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.

README.md

@@ -156,6 +156,7 @@ Staff-only dashboard views expose the same operational state through Django temp
 Host pages include a safe snapshot discovery action that records existing snapshots into SQL.
 Host pages also include a read-only SQL retention plan view before any destructive pruning action.
 Schedules can be created or updated from host pages using the same SQL-backed scheduler model.
+Host config can be edited from host pages while keeping host identity stable.
 
 The remaining internal engine code still contains reusable backup primitives: