feat: add confidence-based RMP matching with manual review workflow

Replace simple auto-matching with scored candidate generation that
considers department overlap, name uniqueness, and rating volume.
Candidates above 0.85 auto-accept; others require admin approval.

migrations/20260131000000_instructor_rmp_links.sql

@@ -0,0 +1,24 @@
+-- Multi-RMP profile support: allow many RMP profiles per instructor.
+-- Each RMP profile still links to at most one instructor (rmp_legacy_id UNIQUE).
+
+-- 1. Create junction table
+CREATE TABLE instructor_rmp_links (
+    id SERIAL PRIMARY KEY,
+    instructor_id INTEGER NOT NULL REFERENCES instructors(id) ON DELETE CASCADE,
+    rmp_legacy_id INTEGER NOT NULL UNIQUE REFERENCES rmp_professors(legacy_id),
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    created_by BIGINT REFERENCES users(discord_id),
+    source VARCHAR NOT NULL DEFAULT 'manual'  -- 'auto' | 'manual'
+);
+
+CREATE INDEX idx_instructor_rmp_links_instructor ON instructor_rmp_links (instructor_id);
+
+-- 2. Migrate existing matches
+INSERT INTO instructor_rmp_links (instructor_id, rmp_legacy_id, source)
+SELECT id, rmp_professor_id,
+       CASE rmp_match_status WHEN 'auto' THEN 'auto' ELSE 'manual' END
+FROM instructors
+WHERE rmp_professor_id IS NOT NULL;
+
+-- 3. Drop old column (and its unique constraint)
+ALTER TABLE instructors DROP COLUMN rmp_professor_id;