store: collapse migrations into a single schema.sql, drop legacy-owner path

Pre-production app, no need to preserve incremental migration history:
replace the 26 migration files with one current-state schema.sql (the
schema.sql comments are now the living documentation), simplify db.go to
apply it once instead of tracking/rebuilding through schema_migrations,
and make user_id NOT NULL everywhere now that there's no staged migration
to accommodate a nullable backfill window.

This removes the reason ClaimLegacyOwner/GENIUSRUN_LEGACY_OWNER_OIDC_SUB
existed (binding a pre-existing singleton-schema database to one account
across a staged migration), so that whole path is gone too -- the
existing dev database was wiped and reseeded fresh under the new schema.

Add cmd/dumpschema, which regenerates docs/DATABASE.md straight from the
live schema (via store.Open + sqlite_master introspection) so the
database documentation can never drift out of sync with reality.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/cmd/geniusrund/main.go

@@ -31,12 +31,6 @@ func main() {
 	}
 	defer db.Close()
 
-	if cfg.LegacyOwnerOIDCSub != "" {
-		if err := db.ClaimLegacyOwner(context.Background(), cfg.LegacyOwnerOIDCSub); err != nil {
-			log.Fatalf("claim legacy owner: %v", err)
-		}
-	}
-
 	authVerifier, err := auth.NewOIDCVerifier(context.Background(), auth.OIDCConfig{
 		IssuerURL:    cfg.OIDCIssuerURL,
 		ClientID:     cfg.OIDCClientID,