Scaffold backend (FastAPI/uv) and frontend (SvelteKit/bun)

Backend: FastAPI app with health check, AppError hierarchy, stderr
logging via LOG_LEVEL, ruff+pyright(strict) config, pytest passing.

Frontend: SvelteKit (TS) with prettier, eslint, vitest (unit +
component), adapter-node for self-hosted deployment on Zeus.

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

backend/app/errors.py

@@ -0,0 +1,41 @@
+"""Base exception hierarchy for the Sagaphone backend.
+
+Errors propagate normally; only the API edge (see main.py's exception
+handler) catches them and translates them into an HTTP response.
+"""
+
+from __future__ import annotations
+
+
+class AppError(Exception):
+    """Base class for all application errors."""
+
+    status_code: int = 500
+
+    def __init__(self, message: str) -> None:
+        super().__init__(message)
+        self.message = message
+
+
+class NotFoundError(AppError):
+    """Requested resource does not exist."""
+
+    status_code = 404
+
+
+class ConfigError(AppError):
+    """Missing or invalid configuration (e.g. secrets.toml)."""
+
+    status_code = 500
+
+
+class NarrationError(AppError):
+    """The narration (Haiku) call failed or returned something unusable."""
+
+    status_code = 502
+
+
+class TTSError(AppError):
+    """The TTS (Piper) synthesis step failed."""
+
+    status_code = 502