/-/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>

datasette/app.py

@@ -58,6 +58,9 @@ from .views.special import (
     PermissionRulesView,
     PermissionCheckView,
     TablesView,
+    InstanceSchemaView,
+    DatabaseSchemaView,
+    TableSchemaView,
 )
 from .views.table import (
     TableInsertView,
@@ -1910,6 +1913,10 @@ class Datasette:
             TablesView.as_view(self),
             r"/-/tables(\.(?P<format>json))?$",
         )
+        add_route(
+            InstanceSchemaView.as_view(self),
+            r"/-/schema(\.(?P<format>json|md))?$",
+        )
         add_route(
             LogoutView.as_view(self),
             r"/-/logout$",
@@ -1951,6 +1958,10 @@ class Datasette:
             r"/(?P<database>[^\/\.]+)(\.(?P<format>\w+))?$",
         )
         add_route(TableCreateView.as_view(self), r"/(?P<database>[^\/\.]+)/-/create$")
+        add_route(
+            DatabaseSchemaView.as_view(self),
+            r"/(?P<database>[^\/\.]+)/-/schema(\.(?P<format>json|md))?$",
+        )
         add_route(
             wrap_view(QueryView, self),
             r"/(?P<database>[^\/\.]+)/-/query(\.(?P<format>\w+))?$",
@@ -1975,6 +1986,10 @@ class Datasette:
             TableDropView.as_view(self),
             r"/(?P<database>[^\/\.]+)/(?P<table>[^\/\.]+)/-/drop$",
         )
+        add_route(
+            TableSchemaView.as_view(self),
+            r"/(?P<database>[^\/\.]+)/(?P<table>[^\/\.]+)/-/schema(\.(?P<format>json|md))?$",
+        )
         add_route(
             RowDeleteView.as_view(self),
             r"/(?P<database>[^\/\.]+)/(?P<table>[^/]+?)/(?P<pks>[^/]+?)/-/delete$",