fix: Add DuckDB examples URI to ECS task definition for showtime environments

This adds the missing SUPERSET__SQLALCHEMY_EXAMPLES_URI environment variable
to the ECS task definition used by showtime ephemeral environments.

Without this configuration, showtime environments were unable to load examples
because the container didn't know where to find the DuckDB examples file that
was downloaded during Docker build (via LOAD_EXAMPLES_DUCKDB=true).

The URI matches what's used in docker-compose.yml and docker-compose-light.yml,
with read-only access mode for safety in ephemeral environments.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

.github/workflows/ecs-task-definition.json

@@ -31,6 +31,10 @@
                 {
                     "name": "TALISMAN_ENABLED",
                     "value": "False"
+                },
+                {
+                    "name": "SUPERSET__SQLALCHEMY_EXAMPLES_URI",
+                    "value": "duckdb:////app/data/examples.duckdb?access_mode=read_only"
                 }
             ],
             "mountPoints": [],