Add configuration-based recommendation ignore rules

[yassinbahri]

Goal

Let teams suppress reviewed recommendations without hiding unrelated candidates.

Proposed design

Define stable recommendation fingerprints from database alias, schema, table, and ordered columns. Read an ignore list from Django settings and omit matching candidates.

Acceptance criteria

• Fingerprints are deterministic and documented.
• Malformed settings produce an actionable Django system check or command error.
• Text and JSON output report how many candidates were ignored.
• Unit tests do not require PostgreSQL.