v1.2.0

New capability — Postgres/Supabase sources in named connections are now runnable (#14)

Previously, a platform: postgres/supabase source under connections: compiled but failed at run time (no credentials on the foreign server). Now the FDW bridge emits a CREATE USER MAPPING, and run injects the source user/password from .df-credentials.json's connections map at execution time.

• Compile emits non-secret placeholders (${SA_CONN:<conn>:user|password}) — secrets never enter the compiled graph or --dry-run/--json output.
• Run substitutes them at the execution choke point, with fail-fast validation if a connection's creds are missing.
• BigQuery sources (non-secret saKeyId path) are unchanged.
• Verified with core unit tests, a CLI substitution unit test, and a live-Postgres integration test (loopback postgres_fdw reading through the foreign table).

For Postgres/Supabase sources: put non-secret host/port/database in workflow_settings.yaml's connection, and the secret user/password under connections.<name> in .df-credentials.json.