feat: add platform singleton flag to metaschema_public.database

[pyramation]

Summary

Upstreams the metaschema_public.database.platform flag from constructive-io/constructive-db#2093, keeping the two copies of metaschema-schema in sync.

platform boolean NOT NULL DEFAULT false,
...
CREATE UNIQUE INDEX databases_database_platform_singleton_idx
  ON metaschema_public.database (platform)
  WHERE platform;

The flag marks the singleton platform (constructive) database — write-once semantics (first setter wins; a trigger in constructive-db's metaschema_private makes it immutable once true). metaschema_private.platform_database_id() in constructive-db is the canonical runtime lookup.

Verify now also asserts the partial unique index; compiled sql/metaschema-schema--0.31.0.sql regenerated via pgpm package (replaces the stale 0.15.5 artifact and syncs Makefile/.control versions).

Link to Devin session: https://app.devin.ai/sessions/7751bd84d4a84873a328def2ec581cba
Requested by: @pyramation

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