feat: parse version prefix from migration filenames (#33)

Closes #28

Migration filenames now follow the pattern `<version>_<description>.sql` (e.g. `001_initial_schema.sql`). The version stored in `schema_migrations` is the numeric prefix only, not the full filename stem.

## Changes

- **`ParseMigrationVersion()`** — new exported function that extracts the numeric prefix from migration filenames. Validates that the prefix is purely numeric and rejects malformed filenames (empty prefix, non-numeric characters, leading underscore).
- **Renamed `001.sql` → `001_initial_schema.sql`** — migration files can now have descriptive names while the tracked version remains `001`. This is safe pre-1.0.0 (no installed base).
- **Deduplicated migration logic** — `runMigrations()` and `ApplyMigrations()` now share a single `applyMigrations()` implementation, plus extracted `collectMigrations()` and `ensureMigrationsTable()` helpers.
- **Unit tests** — `TestParseMigrationVersion` covers valid patterns (version-only, with description, multi-digit, multiple underscores) and error cases (empty, leading underscore, non-numeric, mixed alphanumeric). `TestApplyMigrations` and `TestApplyMigrationsIdempotent` verify end-to-end migration application against an in-memory SQLite database.

Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #33
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/imgcache/stats_test.go

@@ -16,7 +16,7 @@ func setupStatsTestDB(t *testing.T) *sql.DB {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if err := database.ApplyMigrations(db); err != nil {
+	if err := database.ApplyMigrations(context.Background(), db, nil); err != nil {
 		t.Fatal(err)
 	}
 	t.Cleanup(func() { db.Close() })

internal/imgcache/testutil_test.go

@@ -2,6 +2,7 @@ package imgcache
 
 import (
 	"bytes"
+	"context"
 	"database/sql"
 	"image"
 	"image/color"
@@ -193,7 +194,7 @@ func setupServiceTestDB(t *testing.T) *sql.DB {
 	}
 
 	// Use the real production schema via migrations
-	if err := database.ApplyMigrations(db); err != nil {
+	if err := database.ApplyMigrations(context.Background(), db, nil); err != nil {
 		t.Fatalf("failed to apply migrations: %v", err)
 	}