Wizard: don't clobber pre-existing dest tables.

If the dest table already exists, introspect its columns and verify the
wizard's picks line up. Missing columns surface a specific error message
naming what's missing instead of the opaque "column X does not exist"
from a failed COMMENT. On match, skip CREATE + COMMENT so existing
schema and comments aren't touched; staging still gets provisioned.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

pipekit/web/app.py

@@ -436,11 +436,31 @@ async def wizard_create(request: Request):
     effective_staging = staging_table or f"pipekit_staging.{module_name}"
     staging_schema, _, _ = effective_staging.partition(".")
 
+    # If the dest table already exists, don't clobber it. Verify the picks
+    # match its shape and skip CREATE/COMMENT.
+    try:
+        existing_cols = _existing_dest_columns(
+            dest_conn, dest_schema, dest_table_bare)
+    except jrunner.JrunnerError as e:
+        raise HTTPException(500, f"could not introspect dest: {e}")
+    dest_exists = existing_cols is not None
+    if dest_exists:
+        missing = [c["dest_name"] for c in chosen
+                   if c["dest_name"].lower() not in existing_cols]
+        if missing:
+            raise HTTPException(
+                400,
+                f"dest table {qualified_dest} already exists but is missing "
+                f"columns: {', '.join(missing)}. Drop the table, choose a "
+                f"different dest_table, or align your column picks to match "
+                f"the existing schema.")
+
     try:
         jrunner.run_dest_sql(
             dest_conn,
             f"CREATE SCHEMA IF NOT EXISTS {dest_drv.quote_identifier(dest_schema)};",
         )
+        if not dest_exists:
             jrunner.run_dest_sql(dest_conn, create_table_sql)
             comment_sql = _build_comment_sql(dest_drv, qualified_dest,
                                               dest_description, chosen)
@@ -482,6 +502,21 @@ def _sql_str(v: str) -> str:
     return "'" + v.replace("'", "''") + "'"
 
 
+def _existing_dest_columns(dest_conn: dict, schema: str,
+                           table: str) -> set[str] | None:
+    """Return lowercase column names of an existing PG dest table, or
+    None if it doesn't exist. PG-only; fine while pg is the sole dest."""
+    r = jrunner.run_dest_sql(
+        dest_conn,
+        f"SELECT column_name FROM information_schema.columns "
+        f"WHERE table_schema={_sql_str(schema)} "
+        f"AND table_name={_sql_str(table)}",
+    )
+    if not r.rows:
+        return None
+    return {row[0].strip().lower() for row in r.rows if row and row[0]}
+
+
 def _build_comment_sql(dest_drv, qualified_dest: str,
                         table_description: str | None,
                         columns: list[dict]) -> str: