docs: add incident postmortem template

[artylobos]

Summary

• add a public incident postmortem template with required impact, timeline, root-cause, resolution, prevention, and transparency sections
• document the postmortemUrl linking flow for incident records and transparency views
• add an IncidentService helper plus regression test for the canonical postmortem path/label guidance

For #524.

Validation

• npm ci --ignore-scripts --no-audit --no-fund in server/
• npx prisma generate --schema prisma/schema.prisma in server/
• npm test -- --runTestsByPath src/__tests__/incidentService.test.ts --runInBand -> 5 passed
• npx eslint src/services/incidentService.ts src/__tests__/incidentService.test.ts
• npx tsc --noEmit --pretty false --skipLibCheck --target es2020 --module commonjs --moduleResolution node --esModuleInterop --types jest,node src/services/incidentService.ts src/__tests__/incidentService.test.ts
• docs link smoke check
• git diff --check

Note: full npm run build currently fails on existing unrelated repository TypeScript errors, including missing test modules and older strategyRotation/treasury/vaultMetadata test typing errors. The touched IncidentService files pass the focused typecheck above.

No secrets, keys, private logs, or private/system context included.

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