feat(learning): add persistent achievement records

New achievements table: stage_completed/curriculum_completed/assessment_completed/
capstone_completed, idempotent (WHERE NOT EXISTS guard, not a unique-index dance — simple and
correct at this write volume). Not a certificate — a durable internal record Phase 4/5 can
render one from later. recordAssessmentAchievement only fires on an actual pass, defensively,
even though today's only completion_rule already guarantees that.

platform/api/migrations/014_achievements.sql

@@ -0,0 +1,19 @@
+-- Persistent completion records — stage/curriculum/assessment/capstone. Not a certificate
+-- (Phase 4/5 territory, see docs/PHASE3B_LEARNING_AND_ASSESSMENT.md): just the durable fact
+-- that something was achieved, with enough context to render a certificate from later without
+-- re-deriving it from raw attempt history.
+
+CREATE TABLE IF NOT EXISTS achievements (
+  achievement_id           BIGSERIAL PRIMARY KEY,
+  student_id                 TEXT NOT NULL REFERENCES students(student_id),
+  achievement_type           TEXT NOT NULL CHECK (achievement_type IN
+                               ('stage_completed', 'curriculum_completed', 'assessment_completed', 'capstone_completed')),
+  curriculum_path_id         TEXT,
+  curriculum_version         INTEGER,
+  stage_id                    TEXT,
+  assessment_session_id       UUID REFERENCES assessment_sessions(assessment_session_id),
+  achieved_at                 TIMESTAMPTZ NOT NULL DEFAULT now(),
+  metadata                    JSONB
+);
+
+CREATE INDEX IF NOT EXISTS idx_achievements_student ON achievements(student_id, achievement_type);