feat: add local RPM repository with on-demand repodata

Upload RPMs to local repos. Metadata is parsed async after upload
using cavaliergopher/rpm and stored in rpm_metadata table. Repodata
(repomd.xml, primary.xml.gz, filelists.xml.gz, other.xml.gz) is
generated on-demand from the DB — nothing stored in S3.

- RPM provider implements LocalUploader (validates .rpm extension,
  stores under Packages/)
- RPM provider implements PostUploadHook (async goroutine parses RPM
  headers, extracts name/version/arch/deps/etc into rpm_metadata)
- RPM provider implements LocalIndexer (serves repodata/* paths by
  querying rpm_metadata and generating XML on the fly)
- New provider interfaces: PostUploadHook, BlobReader, MetadataStore,
  RPMMetadataReader
- New rpm_metadata table with JSONB columns for requires/provides/
  files/changelogs

Tested e2e: upload cowsay RPM → repodata generated → dnf install
from local repo

internal/database/postgres.go

@@ -124,6 +124,37 @@ func (db *DB) migrate() error {
 		CREATE INDEX IF NOT EXISTS idx_access_log_remote_time ON access_log(remote_name, created_at);
 
 		ALTER TABLE remotes ADD COLUMN IF NOT EXISTS repo_type TEXT DEFAULT 'remote';
+
+		CREATE TABLE IF NOT EXISTS rpm_metadata (
+			id              BIGSERIAL PRIMARY KEY,
+			repo_name       TEXT NOT NULL,
+			file_path       TEXT NOT NULL,
+			content_hash    TEXT NOT NULL,
+			name            TEXT NOT NULL,
+			epoch           INTEGER DEFAULT 0,
+			version         TEXT NOT NULL,
+			release         TEXT NOT NULL,
+			arch            TEXT NOT NULL,
+			summary         TEXT DEFAULT '',
+			description     TEXT DEFAULT '',
+			rpm_size        BIGINT DEFAULT 0,
+			installed_size  BIGINT DEFAULT 0,
+			license         TEXT DEFAULT '',
+			vendor          TEXT DEFAULT '',
+			build_group     TEXT DEFAULT '',
+			build_host      TEXT DEFAULT '',
+			source_rpm      TEXT DEFAULT '',
+			url             TEXT DEFAULT '',
+			packager        TEXT DEFAULT '',
+			requires        JSONB DEFAULT '[]',
+			provides        JSONB DEFAULT '[]',
+			files           JSONB DEFAULT '[]',
+			changelogs      JSONB DEFAULT '[]',
+			created_at      TIMESTAMPTZ DEFAULT NOW(),
+			UNIQUE(repo_name, file_path)
+		);
+
+		CREATE INDEX IF NOT EXISTS idx_rpm_metadata_repo ON rpm_metadata(repo_name);
 	`)
 	return err
 }