Add active-status index for toolchain images

Add a new immutable SQL migration creating a partial index on `installed_toolchain_images(status)` for `installed` and `pending` rows. This aligns with `list_active_images` lookups used during hosted-runner Docker reconnect/prewarm cycles and keeps the query scalable as table size grows, while remaining a defensive optimization at current low cardinality.

Author: BotCoder254

backend/migrations/20260718000002_installed_toolchain_images_status_index.sql

@@ -0,0 +1,14 @@
+-- Partial index matching `list_active_images` (db/toolchain_images.rs), which
+-- runs on every hosted-runner Docker reconnect (30-60 s during an outage) to
+-- build the prewarm set. A partial index over just the active rows keeps that
+-- lookup index-only regardless of table size.
+--
+-- Note: today the table is bounded to the toolchain catalog (toolchain_key is
+-- the primary key and only catalog keys are ever inserted, so ~10 rows max), so
+-- the planner may still seq-scan it — the index is defensive/forward-looking,
+-- not a hot-path necessity at current cardinality. Added as its own migration
+-- because migrations are immutable once applied (editing the create-table
+-- migration would fail sqlx's startup checksum check).
+CREATE INDEX IF NOT EXISTS installed_toolchain_images_active_idx
+    ON installed_toolchain_images (status)
+    WHERE status IN ('installed', 'pending');