fix: "error [object Object]" on API validation errors + SQLite migration

Two fixes:

1. Frontend: FastAPI returns validation errors as {"detail": [{loc, msg}...]}
(array of objects), not a string. The old code did `new Error(detail)` which
stringified the array as "[object Object]". New extractDetail() function
handles both string and array formats, producing readable messages like
"body → slug : field required".

2. Backend: add SQLite migration for the new supports_vision column on
model_configs. Since SQLAlchemy create_all doesn't ALTER existing tables,
existing databases on HuggingFace would crash. The migration runs at
startup and adds the column with DEFAULT 1 (true) if missing.

https://claude.ai/code/session_01Sxmf2zTTcjaQeZXCeXaCxr

backend/app/main.py

@@ -23,6 +23,19 @@ from app.models.database import Base, engine
 logger = logging.getLogger(__name__)
 
 
+def _migrate_model_configs(connection) -> None:
+    """Ajoute la colonne supports_vision si absente (migration BDD existantes)."""
+    from sqlalchemy import inspect, text
+
+    inspector = inspect(connection)
+    columns = {c["name"] for c in inspector.get_columns("model_configs")}
+    if "supports_vision" not in columns:
+        connection.execute(
+            text("ALTER TABLE model_configs ADD COLUMN supports_vision BOOLEAN NOT NULL DEFAULT 1")
+        )
+        logger.info("Migration : colonne supports_vision ajoutée à model_configs")
+
+
 @asynccontextmanager
 async def lifespan(application: FastAPI):
     """Crée les tables SQLite au démarrage, libère l'engine à l'arrêt."""
@@ -52,6 +65,9 @@ async def lifespan(application: FastAPI):
 
     async with engine.begin() as conn:
         await conn.run_sync(Base.metadata.create_all)
+        # Migration : ajouter supports_vision aux model_configs existantes
+        # (create_all ne fait pas d'ALTER TABLE sur les tables existantes)
+        await conn.run_sync(_migrate_model_configs)
     logger.info("Tables SQLite initialisées")
     yield
     await engine.dispose()

frontend/src/lib/api.ts

@@ -190,6 +190,28 @@ export interface CorpusProfile {
 
 // ── Fetch helpers ─────────────────────────────────────────────────────────────
 
+/**
+ * Extract a human-readable error message from a FastAPI error response.
+ * FastAPI may return { detail: "string" } or { detail: [{loc, msg, type}, ...] }
+ * for validation errors (422). This function handles both cases.
+ */
+function extractDetail(payload: unknown, fallback: string): string {
+  if (!payload || typeof payload !== 'object') return fallback
+  const detail = (payload as Record<string, unknown>).detail
+  if (typeof detail === 'string') return detail
+  if (Array.isArray(detail)) {
+    // FastAPI validation error: [{loc: [...], msg: "...", type: "..."}]
+    const messages = detail
+      .map((e: Record<string, unknown>) => {
+        const loc = Array.isArray(e.loc) ? e.loc.join(' → ') : ''
+        return loc ? `${loc} : ${e.msg}` : String(e.msg ?? '')
+      })
+      .filter(Boolean)
+    return messages.length > 0 ? messages.join(' ; ') : fallback
+  }
+  return fallback
+}
+
 async function get<T>(path: string): Promise<T> {
   const resp = await fetch(`${BASE_URL}${path}`)
   if (!resp.ok) throw new Error(`HTTP ${resp.status} — ${path}`)
@@ -204,8 +226,7 @@ async function post<T>(path: string, body?: unknown): Promise<T> {
   })
   if (!resp.ok) {
     const payload = await resp.json().catch(() => null)
-    const detail = (payload as { detail?: string } | null)?.detail
-    throw new Error(detail ?? `HTTP ${resp.status} — ${path}`)
+    throw new Error(extractDetail(payload, `HTTP ${resp.status} — ${path}`))
   }
   return resp.json() as Promise<T>
 }
@@ -218,8 +239,7 @@ async function put<T>(path: string, body?: unknown): Promise<T> {
   })
   if (!resp.ok) {
     const payload = await resp.json().catch(() => null)
-    const detail = (payload as { detail?: string } | null)?.detail
-    throw new Error(detail ?? `HTTP ${resp.status} — ${path}`)
+    throw new Error(extractDetail(payload, `HTTP ${resp.status} — ${path}`))
   }
   return resp.json() as Promise<T>
 }
@@ -228,8 +248,7 @@ async function del(path: string): Promise<void> {
   const resp = await fetch(`${BASE_URL}${path}`, { method: 'DELETE' })
   if (!resp.ok) {
     const payload = await resp.json().catch(() => null)
-    const detail = (payload as { detail?: string } | null)?.detail
-    throw new Error(detail ?? `HTTP ${resp.status} — ${path}`)
+    throw new Error(extractDetail(payload, `HTTP ${resp.status} — ${path}`))
   }
 }
 
@@ -237,8 +256,7 @@ async function postForm<T>(path: string, data: FormData): Promise<T> {
   const resp = await fetch(`${BASE_URL}${path}`, { method: 'POST', body: data })
   if (!resp.ok) {
     const payload = await resp.json().catch(() => null)
-    const detail = (payload as { detail?: string } | null)?.detail
-    throw new Error(detail ?? `HTTP ${resp.status} — ${path}`)
+    throw new Error(extractDetail(payload, `HTTP ${resp.status} — ${path}`))
   }
   return resp.json() as Promise<T>
 }