flanner 0.4.0

Added

• Schema migration runner: a versioned MIGRATIONS registry upgrades an existing
  database in-place when SCHEMA_VERSION rises, instead of only stamping the
  version (see docs/adr/0003-schema-migrations.md)

Changed

• Extracted the shared plan write-path (frontmatter, filename, save, hash,
  version record) into flanner/plan_ops.write_version; the MCP server and web
  UI both call it so the four create/update copies cannot drift

Fixed

• Bump pytest to >=9.0.3 so pip-audit passes (PYSEC-2026-1845)
• Replace deprecated datetime.utcnow() with a naive-UTC helper
• Refresh the stale docs/INSTALLATION.md

Security

• flanner web warns when binding a non-local host (the web UI has no auth)