feat: add artifact_id indexes on release_tags and image_comments

scripts/sql/37504600_add_artifact_tagging_indexes.down.sql

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+-- Rollback: drop indexes added by 37504600_add_artifact_tagging_indexes.up.sql
+
+DROP INDEX IF EXISTS idx_release_tags_artifact_id;
+DROP INDEX IF EXISTS idx_image_comments_artifact_id;

scripts/sql/37504600_add_artifact_tagging_indexes.up.sql

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+-- WS-G: add indexes for unindexed artifact_id lookups on release_tags/image_comments
+-- to eliminate sequential scans on the following user-facing hot paths:
+--   * ImageTaggingRepositoryImpl.GetTagsByArtifactId
+--     (internal/sql/repository/imageTagging/ImageTaggingRepository.go)
+--     — called from ImageTaggingService/ImageTaggingReadService when viewing/editing an
+--     artifact's release tags.
+--   * ImageTaggingRepositoryImpl.GetImageComment / GetImageCommentsByArtifactIds
+--     (same file) — called from ImageTaggingService when rendering an artifact's comment
+--     (build/deployment history rows).
+--
+-- Note: CREATE INDEX CONCURRENTLY cannot run inside a transaction (golang-migrate wraps
+-- each migration in one). We use plain CREATE INDEX IF NOT EXISTS here so the migration is
+-- idempotent, matching this repo's existing convention (see 36304600_add_ci_artifact_query_indexes).
+-- For large existing tables, operators SHOULD create these indexes manually with
+-- CONCURRENTLY BEFORE deploying this release, to avoid blocking writes during pod startup:
+--
+--   CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_release_tags_artifact_id
+--     ON release_tags (artifact_id);
+--   CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_image_comments_artifact_id
+--     ON image_comments (artifact_id);
+--
+-- Once the indexes exist, this migration is a no-op (IF NOT EXISTS).
+
+-- release_tags.artifact_id — the table's only existing index is UNIQUE(app_id, tag_name),
+-- which doesn't help an artifact_id-only lookup.
+CREATE INDEX IF NOT EXISTS idx_release_tags_artifact_id
+  ON release_tags (artifact_id);
+
+-- image_comments.artifact_id — no index beyond the primary key today.
+CREATE INDEX IF NOT EXISTS idx_image_comments_artifact_id
+  ON image_comments (artifact_id);