feat(all): add satellite_url field with auto-detection and first-heartbeat updates

Add satellite_url field to satellites table to track publicly accessible URLs.
Supports explicit configuration via DEPLOYSTACK_SATELLITE_URL environment
variable or auto-detection from request headers (X-Forwarded-Proto/Host).

Backend changes:
- Add satellite_url field to satellites schema (NOT NULL)
- Create idempotent migration with IF NOT EXISTS handling
- Update registration endpoint to accept and auto-detect satellite URLs
- Update heartbeat endpoint to accept URL on first heartbeat only
- Add detectSatelliteUrl() helper with reverse proxy header support

Satellite changes:
- Update client to send URL during registration
- Add hasUpdatedUrlThisSession flag to HeartbeatService
- Send URL only on first heartbeat after startup (not every 30s)
- Support explicit URL from env var or empty string for auto-detection

Documentation updates:
- Update satellite developer docs (registration.mdx, backend-communication.mdx)
- Update backend developer docs (communication.mdx)
- Document auto-detection behavior in .env.example

URL updates occur during:
- Initial satellite registration
- First heartbeat after satellite restart

If DEPLOYSTACK_SATELLITE_URL not set, backend auto-detects from request.

Author: Lasim

services/backend/drizzle/migrations/0011_lush_cable.sql

@@ -0,0 +1,3 @@
+ALTER TABLE "satellites" ADD COLUMN IF NOT EXISTS "satellite_url" text NOT NULL DEFAULT 'http://unknown';
+--> statement-breakpoint
+ALTER TABLE "satellites" ALTER COLUMN "satellite_url" DROP DEFAULT;

services/backend/drizzle/migrations/meta/0011_lush_cable.json

@@ -0,0 +1,14 @@
+{
+  "id": "f8c5e2a1-b3d4-4c9e-8f7a-6b5c4d3e2f1a",
+  "prevId": "0010_young_jimmy_woo",
+  "version": "7",
+  "dialect": "postgresql",
+  "tables": {},
+  "enums": {},
+  "schemas": {},
+  "_meta": {
+    "schemas": {},
+    "tables": {},
+    "columns": {}
+  }
+}