Fix version display: show actual version instead of 'vunknown'

Root cause: Dockerfile build context is ./app/ but VERSION file is at
repo root, so it's excluded from the Docker image. The context processor
tried parent.parent.parent which resolves to / inside the container.

Fix:
- Context processor now checks APP_VERSION env var first, then tries
  multiple file paths (repo root for local dev, app/ dir for Docker)
- Dockerfile accepts APP_VERSION build arg and sets it as ENV
- compose.yml passes APP_VERSION build arg to all service builds

Note: Deploy script needs `export APP_VERSION=$(cat VERSION)` before
docker-compose build for the build arg to pick up the version.

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

compose.yml

@@ -25,7 +25,10 @@ services:
     image: redis:7-alpine
 
   web:
-    build: ./app
+    build:
+      context: ./app
+      args:
+        APP_VERSION: ${APP_VERSION:-unknown}
     depends_on:
       db:
         condition: service_healthy
@@ -62,7 +65,10 @@ services:
     command: ["gunicorn", "core.wsgi:application", "--bind", "0.0.0.0:8000", "--workers", "3"]
 
   worker:
-    build: ./app
+    build:
+      context: ./app
+      args:
+        APP_VERSION: ${APP_VERSION:-unknown}
     environment:
       - POSTGRES_DB=${POSTGRES_DB}
       - POSTGRES_USER=${POSTGRES_USER}
@@ -88,7 +94,10 @@ services:
     command: ["celery", "-A", "core", "worker", "-l", "info"]
 
   beat:
-    build: ./app
+    build:
+      context: ./app
+      args:
+        APP_VERSION: ${APP_VERSION:-unknown}
     environment:
       - POSTGRES_DB=${POSTGRES_DB}
       - POSTGRES_USER=${POSTGRES_USER}
@@ -114,7 +123,10 @@ services:
     command: ["celery", "-A", "core", "beat", "-l", "info"]
 
   mcp:
-    build: ./app
+    build:
+      context: ./app
+      args:
+        APP_VERSION: ${APP_VERSION:-unknown}
     depends_on:
       db:
         condition: service_healthy