Switch orchestrator_config from a generic key/text store to a typed
single-row table with a dedicated teardown_timeout_seconds REAL column.
The CHECK (id = 1) constraint enforces at most one config row.
Remove TEARDOWN_TIMEOUT_KEY constant; update OrchestratorConfigStore to
expose get/set/delete_teardown_timeout_seconds() instead of the generic
get/set/delete(key). Resolving the DB value in resolve_teardown_timeout()
no longer needs a float() conversion since the column is already REAL.
Resolves via ENV VAR -> orchestrator DB config -> default (30 s, up from 5 s):
BOT_BOTTLE_ORCHESTRATOR_TEARDOWN_TIMEOUT_SECONDS
teardown_timeout_seconds key in new orchestrator_config table (bot-bottle.db)
New OrchestratorConfigStore (same DbStore/TableMigrations pattern as the
registry) stores the DB-level setting. resolve_teardown_timeout() implements
the priority chain and is called at stack.callback registration time in all
three backends (macos_container, docker, firecracker).