feat(dashboard): Phase 13 — Executive Dashboard

PostureSnapshot model, Alembic migration (b039exec), schemas, service
aggregating all phases (coverage/risk/operations/knowledge/MTTD), and
router at /api/v1/dashboard with executive view, KPIs, coverage-by-tactic,
posture-history, posture-snapshot, and activity-feed endpoints.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/alembic/versions/b039_executive_dashboard.py

@@ -0,0 +1,77 @@
+"""Phase 13: Executive Dashboard — posture_snapshots table.
+
+Revision ID: b039exec
+Revises:     b038risk
+Create Date: 2026-05-20
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+revision       = "b039exec"
+down_revision  = "b038risk"
+branch_labels  = None
+depends_on     = None
+
+
+def upgrade() -> None:
+    conn = op.get_bind()
+    conn.execute(sa.text("""
+        CREATE TABLE IF NOT EXISTS posture_snapshots (
+            id              UUID         PRIMARY KEY DEFAULT gen_random_uuid(),
+            snapshot_date   DATE         NOT NULL,
+
+            -- Coverage
+            total_techniques   INTEGER NOT NULL DEFAULT 0,
+            validated_count    INTEGER NOT NULL DEFAULT 0,
+            partial_count      INTEGER NOT NULL DEFAULT 0,
+            not_covered_count  INTEGER NOT NULL DEFAULT 0,
+            coverage_pct       FLOAT   NOT NULL DEFAULT 0.0,
+
+            -- Risk
+            avg_risk_score     FLOAT   NOT NULL DEFAULT 0.0,
+            critical_count     INTEGER NOT NULL DEFAULT 0,
+            high_count         INTEGER NOT NULL DEFAULT 0,
+            medium_count       INTEGER NOT NULL DEFAULT 0,
+            low_count          INTEGER NOT NULL DEFAULT 0,
+
+            -- Operations
+            open_queue_items   INTEGER NOT NULL DEFAULT 0,
+            orphan_techniques  INTEGER NOT NULL DEFAULT 0,
+
+            -- Knowledge
+            playbook_count     INTEGER NOT NULL DEFAULT 0,
+            lesson_count       INTEGER NOT NULL DEFAULT 0,
+
+            -- MTTD
+            mttd_avg_seconds   FLOAT,
+            executions_30d     INTEGER NOT NULL DEFAULT 0,
+            detection_rate_30d FLOAT,
+
+            -- Meta
+            created_by  UUID REFERENCES users(id) ON DELETE SET NULL,
+            created_at  TIMESTAMP WITHOUT TIME ZONE DEFAULT now(),
+            extra       JSONB
+        )
+    """))
+
+    # Unique constraint: one snapshot per calendar day
+    conn.execute(sa.text("""
+        DO $$ BEGIN
+            ALTER TABLE posture_snapshots
+                ADD CONSTRAINT uq_posture_snapshot_date UNIQUE (snapshot_date);
+        EXCEPTION WHEN duplicate_table THEN NULL;
+                 WHEN duplicate_object THEN NULL;
+        END $$
+    """))
+
+    # Index for date-range trend queries
+    conn.execute(sa.text("""
+        CREATE INDEX IF NOT EXISTS ix_posture_snapshots_date
+            ON posture_snapshots (snapshot_date)
+    """))
+
+
+def downgrade() -> None:
+    conn = op.get_bind()
+    conn.execute(sa.text("DROP TABLE IF EXISTS posture_snapshots CASCADE"))