/-/schema and /db/-/schema and /db/table/-/schema pages (plus .json/.md)

* Add schema endpoints for databases, instances, and tables

Closes: #2586

This commit adds new endpoints to view database schemas in multiple formats:

- /-/schema - View schemas for all databases (HTML, JSON, MD)
- /database/-/schema - View schema for a specific database (HTML, JSON, MD)
- /database/table/-/schema - View schema for a specific table (JSON, MD)

Features:
- Supports HTML, JSON, and Markdown output formats
- Respects view-database and view-table permissions
- Uses group_concat(sql, ';' || CHAR(10)) from sqlite_master to retrieve schemas
- Includes comprehensive tests covering all formats and permission checks

The JSON endpoints return:
- Instance level: {"schemas": [{"database": "name", "schema": "sql"}, ...]}
- Database level: {"database": "name", "schema": "sql"}
- Table level: {"database": "name", "table": "name", "schema": "sql"}

Markdown format provides formatted output with headings and SQL code blocks.

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

tests/test_html.py

@@ -142,7 +142,7 @@ async def test_database_page(ds_client):
 
     # And a list of tables
     for fragment in (
-        '<h2 id="tables">Tables</h2>',
+        '<h2 id="tables">Tables',
         '<h3><a href="/fixtures/sortable">sortable</a></h3>',
         "<p><em>pk, foreign_key_with_label, foreign_key_with_blank_label, ",
     ):