config: fix media path at /var/thoughtsync (no env knob)

The media/data location is no longer configurable — DATA_DIR is a fixed
constant (/var/thoughtsync) and THOUGHTSYNC_MEDIA_ROOT is removed, so a
mutable path can't drift from where the volume is mounted. media_root() =
DATA_DIR/media. Both compose files drop THOUGHTSYNC_DATA_DIR and mount the
data volume at /var/thoughtsync (was the contradictory /data). conftest
drops the stale MEDIA_ROOT monkeypatch (create_app never reads DATA_DIR).

Net env surface: THOUGHTSYNC_DATABASE_URL (required) + THOUGHTSYNC_SECRET_KEY
(optional break-glass).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

docker-compose.dev.yml

@@ -31,14 +31,13 @@ services:
         condition: service_healthy
     environment:
       THOUGHTSYNC_DATABASE_URL: postgresql+asyncpg://thoughtsync:thoughtsync@db:5432/thoughtsync
-      THOUGHTSYNC_DATA_DIR: /data
       PYTHONPATH: /app/src
     volumes:
       - ./pyproject.toml:/app/pyproject.toml
       - ./alembic.ini:/app/alembic.ini
       - ./alembic:/app/alembic
       - ./src:/app/src
-      - thoughtsync-dev-data:/data
+      - thoughtsync-dev-data:/var/thoughtsync
     ports:
       - "5000:5000"
     # Install deps, run migrations, then serve with live reload on the mounted src.