Move schema_migrations table creation into 000.sql with INTEGER version column

Replace the monolithic schema.sql embed with a numbered migration system
following the pixa pattern:

- Add schema/000.sql: bootstrap migration creating schema_migrations table
  with INTEGER PRIMARY KEY version column and applied_at timestamp
- Move full schema to schema/001.sql (renamed from schema.sql)
- Remove redundant schema/008_uploads.sql (uploads table already in main schema)
- Rewrite database.go migration logic:
  - Embed schema/*.sql directory instead of single schema.sql
  - bootstrapMigrationsTable() checks for table existence, applies 000.sql
  - applyMigrations() iterates numbered files, skips already-applied versions
  - ParseMigrationVersion() extracts version from filenames (e.g. 001.sql)
  - Go code does zero INSERTs for bootstrap — 000.sql is self-contained
- Update database_test.go to verify schema_migrations table exists

internal/database/database_test.go

@@ -26,9 +26,10 @@ func TestDatabase(t *testing.T) {
 		t.Fatal("database connection is nil")
 	}
 
-	// Test schema creation (already done in New)
+	// Test schema creation (already done in New via migrations)
 	// Verify tables exist
 	tables := []string{
+		"schema_migrations",
 		"files", "file_chunks", "chunks", "blobs",
 		"blob_chunks", "chunk_files", "snapshots",
 	}