feat(accounts): home district with 6-month cooldown

- accounts/models.py: add district_changed_at DateTimeField + VALID_DISTRICTS constant (14 Kerala districts)
- migration 0013_user_district_changed_at: nullable DateTimeField, no backfill
- WebRegisterForm: accept optional district during signup, stamp district_changed_at
- UpdateProfileView: enforce 183-day cooldown with human-readable error
- LoginView/WebRegisterView/StatusView: include district_changed_at in responses

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -5,6 +5,19 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.0.0/), version
 
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+## [1.7.0] — 2026-04-04
+
+### Added
+- **Home District with 6-month cooldown**
+  - `district_changed_at` DateTimeField on User model (migration `0013_user_district_changed_at`) — nullable, no backfill; NULL means "eligible to change immediately"
+  - `VALID_DISTRICTS` constant (14 Kerala districts) in `accounts/models.py` for server-side validation
+  - `WebRegisterForm` now accepts optional `district` field; stamps `district_changed_at` on valid selection during signup
+  - `UpdateProfileView` enforces 183-day (~6 months) cooldown — rejects district changes within the window with a human-readable "Next change: {date}" error
+  - `district_changed_at` included in all relevant API responses: `LoginView`, `WebRegisterView`, `StatusView`, `UpdateProfileView`
+  - `StatusView` now also returns `district` field (was previously missing)
+
+---
+
 ## [1.6.2] — 2026-04-03
 
 ### Security