fix(notebooks): api_explorer target URL + zotero_tutorial polars schema inference

- api_explorer pointed at http://localhost:8080 — a host-side dev
  server that never exists inside the notebooks container (nb issue
  #546 alongside the api service wedge). Now defaults to the compose
  service http://api:8000, overridable via STACK_API_URL.
- zotero_tutorial's CASE/GROUP_CONCAT queries can yield columns that
  are all-NULL within polars' default inference window with strings
  appearing later ('could not append value', nb issue #561 — surfaced
  once the immutable=1 fix got reads past the lock). Pin
  infer_schema_length=None on all read_database calls.

Verified headless in the prod container: both notebooks now export
with zero cell errors.

notebooks/api_explorer.py

@@ -14,16 +14,20 @@ def _():
     Interactive test harness for the Stack REST API. Exercises every endpoint
     group: **health**, **pipelines**, **lineage**, **bib**, **schema**, and **auth**.
 
-    Start the API server first: `uv run stack api serve --port 8080`
+    Defaults to the compose-internal `api` service; set `STACK_API_URL` to
+    point elsewhere (e.g. `http://localhost:8080` with a local
+    `uv run stack api serve --port 8080`).
     """)
     return (mo,)
 
 
 @app.cell
 def _():
+    import os
+
     import httpx
 
-    BASE = "http://localhost:8080"
+    BASE = os.environ.get("STACK_API_URL", "http://api:8000")
     client = httpx.Client(base_url=BASE, timeout=30)
     return (client,)
 

notebooks/zotero_tutorial.py

@@ -117,6 +117,7 @@ def _(db, mo, pl):
         ORDER BY c.collectionName
         """,
         connection=db,
+        infer_schema_length=None,
     )
 
     def _build_tree(df, parent_id=None, depth=0):
@@ -168,6 +169,7 @@ def _(db, mo, pl):
         ORDER BY count DESC
         """,
         connection=db,
+        infer_schema_length=None,
     )
 
     mo.ui.table(
@@ -210,6 +212,7 @@ def _(db, mo, pl):
         LIMIT 25
         """,
         connection=db,
+        infer_schema_length=None,
     )
 
     mo.ui.table(recent_df, label="Recent Items") if recent_df.height > 0 else mo.md(
@@ -415,6 +418,7 @@ def _(db, mo, pl):
         ORDER BY count DESC
         """,
         connection=db,
+        infer_schema_length=None,
     )
 
     mo.ui.table(tags_df, label="Tags") if tags_df.height > 0 else mo.md(
@@ -451,6 +455,7 @@ def _(db, mo, pl):
         LIMIT 50
         """,
         connection=db,
+        infer_schema_length=None,
     )
 
     mo.ui.table(creators_df, label="Top Creators") if creators_df.height > 0 else mo.md(
@@ -502,6 +507,7 @@ def _(db, mo, pl):
         ORDER BY i.dateAdded DESC
         """,
         connection=db,
+        infer_schema_length=None,
     )
 
     mo.md(f"Exported **{library_df.height}** items to `library_df`")