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

tests/conftest.py

@@ -5,9 +5,8 @@ from thoughtsync.config import Config
 
 @pytest.fixture(autouse=True)
 def _isolated_data_dir(tmp_path, monkeypatch):
-    """Point DATA_DIR at a writable temp dir so create_app() can generate its
-    signing key without needing /var/thoughtsync to exist (DB-free unit tests)."""
-    data_dir = tmp_path / "data"
-    monkeypatch.setattr(Config, "DATA_DIR", str(data_dir))
-    monkeypatch.setattr(Config, "MEDIA_ROOT", str(data_dir / "media"))
+    """Keep any media writes on an isolated temp dir rather than the fixed
+    /var/thoughtsync. DB-free unit tests never actually hit it (create_app takes its
+    signing key from env-or-random, not a file), but this stays defensive."""
+    monkeypatch.setattr(Config, "DATA_DIR", str(tmp_path / "data"))
     yield