Move the llm_records and llm_usage DDL out of Go constants into embedded
files db/migrations/0001_llm_records.sql and 0002_llm_usage.sql (byte-for-byte
the old constants). Open now applies pending migrations after schema.sql and
before seedBuiltins, under the existing schema advisory lock and on the
lock-holding connection. Each migration runs in its own transaction with its
schema_migrations row (version, name, SHA-256 checksum, applied_at).
Open fails closed when the database has a version missing from the binary, or
an applied migration whose file name or checksum changed. A failing migration
is rolled back and blocks startup; earlier ones stay applied.
EnsureLLMRecordsTable and EnsureLLMUsageTable re-run the embedded files and
keep their transaction-level schema lock; Open never calls them.
