Add central ASN address-group expander

Expand asn address groups to concrete prefixes centrally (one iplocate key,
consistent fleet-wide) and refresh them on a per-group TTL (default 24h). A
background Refresher scans for due groups, unions each group's ASNs to a deduped
prefix set, and writes them to a new resolved/resolved_at column (migration
0002). Fail-safe: a lookup error or empty expansion keeps the last-good set,
never emptying it. The compiler folds resolved prefixes into the rendered set
members; membership churn bumps the generation but never rewrites rules. The
iplocate client is endpoint-configurable and response-tolerant, documented as
needing endpoint/key confirmation. Unit tests cover TTL parsing, due-checks, and
union/dedup/error propagation with a fake expander.

internal/database/migrations/0002_asn_resolution.sql

@@ -0,0 +1,7 @@
+-- Cache expanded ASN prefixes on the address group. `resolved` holds the concrete
+-- CIDRs the expander last produced; `resolved_at` timestamps the last successful
+-- expansion. Membership churn here is deliberately separate from rule definition:
+-- the compiler folds `resolved` into the rendered set without touching rules.
+ALTER TABLE address_groups
+    ADD COLUMN resolved    JSONB       NOT NULL DEFAULT '[]'::jsonb,
+    ADD COLUMN resolved_at TIMESTAMPTZ;