fix(etl): implement Postgres schema check in ensure_manager_similarity_table

[stranske-automation-bot]

Why

In etl/manager_similarity_flow.py:27-47, the function ensure_manager_similarity_table(conn) executes table creation DDL only when isinstance(conn, sqlite3.Connection).

When executed against a PostgreSQL connection, ensure_manager_similarity_table(conn) executes no statements and provides no fallback or table verification. This inconsistency contrasts with all other table ensure functions across the codebase (such as etl/conviction_flow.py, alerts/db.py, and etl/activism_detection.py), which provide PostgreSQL DDL branches.

Scope

• Implement PostgreSQL DDL execution in ensure_manager_similarity_table() in etl/manager_similarity_flow.py:27-47 matching dialect patterns used across etl/.
• Add test coverage in tests/test_manager_similarity_flow.py verifying table creation DDL execution on PostgreSQL connections.

Implementation Notes

• Mirror the PostgreSQL table creation schema in etl/conviction_flow.py using BIGINT, TIMESTAMPTZ, and FLOAT types.
• Ensure idempotent execution using CREATE TABLE IF NOT EXISTS manager_similarity (...).

Non-Goals

• Modifying similarity computation algorithms or vector embedding distance metrics.
• Adding scaffold-only stubs, mocks, or empty bypass functions without wiring up the real implementation does NOT count as done and constitutes a failure of this issue.

Tasks

• Add PostgreSQL dialect DDL branch to function ensure_manager_similarity_table() in etl/manager_similarity_flow.py.
• Add test cases in tests/test_manager_similarity_flow.py asserting PostgreSQL DDL executes on non-SQLite connection instances.

Acceptance Criteria

• Running pytest tests/test_manager_similarity_flow.py executes without errors.
• Calling ensure_manager_similarity_table(pg_conn) on a PostgreSQL mock connection executes DDL creating table manager_similarity.
• Verification gate: deliberately break the implementation by omitting PostgreSQL handling in etl/manager_similarity_flow.py and verify pytest tests/test_manager_similarity_flow.py fails.