feat: add database schema migrations and seed data

8 SQL migration files covering all 10 tables, RLS policies with
SECURITY DEFINER helpers, and seed data for SCW1 site + 3 zones.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AWxyMibCuGGtSQSqfajDQ7

supabase/migrations/20260709000004_evidence_investigations.sql

@@ -0,0 +1,34 @@
+-- supabase/migrations/20260709000004_evidence_investigations.sql
+CREATE TYPE evidence_stage AS ENUM (
+  'report', 'response', 'investigation', 'capa', 'verification'
+);
+
+CREATE TABLE evidence_files (
+  id          UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  incident_id UUID NOT NULL REFERENCES incidents(id) ON DELETE RESTRICT,
+  stage       evidence_stage NOT NULL,
+  file_url    TEXT NOT NULL,
+  file_type   TEXT NOT NULL,
+  file_hash   TEXT NOT NULL,        -- immutable after upload (DOSH audit integrity)
+  uploaded_by UUID NOT NULL REFERENCES users(id),
+  uploaded_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+  deleted     BOOLEAN NOT NULL DEFAULT false  -- NEVER hard-delete; use this flag
+);
+
+CREATE TYPE rca_method AS ENUM ('five_why', 'fishbone', 'other');
+
+CREATE TABLE investigations (
+  id                    UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  incident_id           UUID NOT NULL REFERENCES incidents(id) ON DELETE RESTRICT,
+  investigator_id       UUID NOT NULL REFERENCES users(id),
+  method                rca_method NOT NULL DEFAULT 'five_why',
+  findings_text         TEXT,
+  root_cause_summary    TEXT,
+  alcohol_test_result   TEXT,
+  witness_statement_refs TEXT[],
+  completed_at          TIMESTAMPTZ,
+  created_at            TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX evidence_files_incident_idx  ON evidence_files(incident_id);
+CREATE INDEX investigations_incident_idx  ON investigations(incident_id);