fix: wrap uuid_generate_v7 function in goose StatementBegin/End

goose splits SQL on semicolons and misinterprets dollar-quoted strings
($$...$$) as statement boundaries, causing a parse error on the PL/pgSQL
function body. StatementBegin/End annotations tell goose to treat the
block as a single statement.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/migrations/001_initial_schema.sql

@@ -10,6 +10,7 @@ CREATE EXTENSION IF NOT EXISTS pg_trgm;
 -- UUID v7 generator (time-ordered, millisecond precision).
 -- Structure: 48-bit unix_ts_ms | 4-bit version (0111) | 12-bit rand_a | 2-bit variant (10) | 62-bit rand_b
 -- ---------------------------------------------------------------------------
+-- +goose StatementBegin
 CREATE OR REPLACE FUNCTION uuid_generate_v7()
 RETURNS uuid
 AS $$
@@ -38,6 +39,7 @@ BEGIN
     RETURN encode(uuid_bytes, 'hex')::uuid;
 END
 $$ LANGUAGE plpgsql VOLATILE;
+-- +goose StatementEnd
 
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