Add HACS support

Add hacs.json, README.md, and update manifest.json URLs and codeowner
to support installation via HACS custom repository.

README.md

@@ -0,0 +1,17 @@
+# Room Calendar HA
+
+A Home Assistant custom integration for managing room/rental reservations via CalDAV or the `room_cal` REST API.
+
+## Installation via HACS
+
+1. In HACS, go to **Integrations → Custom repositories**
+2. Add `https://git.zi3.xyz/code/room_cal_ha` with category **Integration**
+3. Install **Room Calendar** from HACS
+4. Restart Home Assistant
+5. Add the integration via **Settings → Devices & Services → Add Integration → Room Calendar**
+
+## Requirements
+
+- Home Assistant 2026.5 or later
+- For CalDAV mode: a CalDAV server with `caldav` + `icalendar` Python packages (installed automatically)
+- For API mode: a running `room_cal` REST API instance

custom_components/room_cal_ha/manifest.json

@@ -3,10 +3,10 @@
   "name": "Room Calendar",
   "version": "0.1.0",
   "config_flow": true,
-  "documentation": "https://github.com/room-cal-ha/room_cal_ha",
-  "issue_tracker": "https://github.com/room-cal-ha/room_cal_ha/issues",
+  "documentation": "https://git.zi3.xyz/code/room_cal_ha",
+  "issue_tracker": "https://git.zi3.xyz/code/room_cal_ha/issues",
   "requirements": ["caldav>=3.2.1", "icalendar>=7.1.3"],
   "dependencies": [],
-  "codeowners": [],
+  "codeowners": ["@zib"],
   "iot_class": "cloud_polling"
 }

hacs.json

@@ -0,0 +1,5 @@
+{
+  "name": "Room Calendar",
+  "render_readme": true,
+  "homeassistant": "2026.5.0"
+}