migrate_json_to_sqlite

Migrate seq_id_to_seq.json -> SQLite (WAL) sequences table.

Table (must exist or will be created if missing):

sequences(

id TEXT PRIMARY KEY, seq TEXT UNIQUE NOT NULL, sha256 TEXT UNIQUE NOT NULL, len INTEGER NOT NULL, created_at TEXT NOT NULL DEFAULT (CURRENT_TIMESTAMP), last_seen_at TEXT NOT NULL DEFAULT (CURRENT_TIMESTAMP), uses_count INTEGER NOT NULL DEFAULT 0

)

Behaviour: - Preserves existing rows, inserts new ones. - If an ID exists with a different sequence, logs a conflict and keeps the DB value. - If a sequence exists but with a different ID, logs a mismatch and keeps the DB value. - New rows get uses_count=0, created_at/last_seen_at=CURRENT_TIMESTAMP.