Add passexec_cmd, passexec_expiration, kerberos_conn, tags, and
post_connection_sql to SharedServer so non-owners get their own
per-user values instead of inheriting the owner's. Drop the unused
db_res column which was never overlaid or writable by non-owners.
Key changes:
- New Alembic migration (sharedserver_feature_parity) adds 5 columns,
drops db_res, cleans up orphaned records. All operations idempotent.
- Overlay copies new fields from SharedServer instead of suppressing
- _owner_only_fields guard blocks non-owners from setting passexec_cmd,
passexec_expiration, db_res, db_res_type via API
- Non-owners can set post_connection_sql (runs under their own creds)
- update_tags and flag_modified use sharedserver for non-owners
- update() response returns sharedserver tags for non-owners
- ServerManager passexec suppression with config.SERVER_MODE guard
- UI: post_connection_sql editable for non-owners (readonly only when
connected, not when shared)
- SCHEMA_VERSION bumped to 51
- Comprehensive unit tests for overlay, write guards, and tag deltas
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