feat(version): /api/version + build-time version/commit stamping

Add a VERSION file (0.1.0) and inject APP_VERSION/GIT_SHA/BUILD_DATE as Docker
build-args (both stages; Vite inlines them into the SPA). New public GET
/api/version returns app_version, git_sha, build_date and the Alembic head as the
database revision. deploy.sh and the localdev build pass the args.

backend/app/routes/version.py

@@ -0,0 +1,23 @@
+from fastapi import APIRouter, Depends
+from sqlalchemy import text
+from sqlalchemy.orm import Session
+
+from app.config import settings
+from app.db import get_db
+
+router = APIRouter(prefix="/api", tags=["version"])
+
+
+@router.get("/version")
+def get_version(db: Session = Depends(get_db)) -> dict:
+    """Build + schema version, for the About dialog and the 'new version' banner.
+    Intentionally public (no auth) and cheap: just env-baked build info plus the current
+    Alembic migration head (the 'database version'). app_version/git_sha/build_date are
+    injected at image build time (see the Dockerfile build-args)."""
+    db_revision = db.execute(text("SELECT version_num FROM alembic_version")).scalar()
+    return {
+        "app_version": settings.app_version,
+        "git_sha": settings.git_sha,
+        "build_date": settings.build_date or None,
+        "db_revision": db_revision,
+    }