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/schema/000.sql

@@ -0,0 +1,9 @@
+-- Migration 000: Schema migrations tracking table
+-- Applied as a bootstrap step before the normal migration loop.
+
+CREATE TABLE IF NOT EXISTS schema_migrations (
+    version INTEGER PRIMARY KEY,
+    applied_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+INSERT OR IGNORE INTO schema_migrations (version) VALUES (0);