Make migration 000007 idempotent

[bakhterets]

Adds audit user fields (created_by, contact_email, version) to incident and
(created_by, modified_by) to incident_status in a re-runnable way.

Changes

• Use ADD COLUMN IF NOT EXISTS for all new columns.
• Backfill version = 1 for existing rows, then apply SET DEFAULT 1 / SET NOT NULL
  in a separate ALTER COLUMN statement.

Why

ADD COLUMN IF NOT EXISTS ... NOT NULL DEFAULT 1 silently skips the constraint when the
column already exists as nullable, leaving the schema out of sync with the GORM model.
Applying the constraint separately guarantees the final state regardless of prior history,
and the backfill must precede SET NOT NULL because it fails on any NULL row.

Verification

Tested on a scratch database:

• clean run 1 → 8: OK, no dirty state
• re-applying 000007 over an existing schema: NOTICE ... skipping, no errors
• goto 6 → up with existing rows: data preserved, version restored to 1
• final schema: version integer NOT NULL DEFAULT 1, others varchar(255) NULL