Move schema_migrations table creation from Go code into 000.sql

The schema_migrations table definition now lives in
internal/database/schema/000.sql instead of being hardcoded as an
inline SQL string in database.go. A bootstrap step checks
sqlite_master for the table and applies 000.sql when it is missing.
Existing databases that already have the table (created by older
inline code) get version 000 back-filled so the normal migration
loop skips the file.

Also deduplicates the migration logic: both the Database.runMigrations
method and the exported ApplyMigrations helper now delegate to a single
applyMigrations function.

Adds database_test.go with tests for fresh migration, idempotency,
bootstrap on a fresh DB, and backwards compatibility with legacy DBs.

internal/database/schema/000.sql

@@ -0,0 +1,9 @@
+-- Migration 000: Schema migrations tracking table
+-- This must be the first migration applied. The bootstrap logic in
+-- database.go applies it directly (bypassing the normal migration
+-- loop) when the schema_migrations table does not yet exist.
+
+CREATE TABLE IF NOT EXISTS schema_migrations (
+    version TEXT PRIMARY KEY,
+    applied_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);