fc-common: add GC pinning and machine health infrastructure

Migration 017 adds `builds.keep`, `jobsets.keep_nr`, and health tracking
columns to `remote_builders`. Repo layer implements `set_keep`,
`list_pinned_ids`, `record_failure` with exponential backoff,
`record_success`, and `find_for_system` filtering of disabled builders.
GC root cleanup now skips pinned builds.

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: Ibba121de3dc42f71204e3a8f5776aa8b6a6a6964

crates/common/migrations/017_gc_pinning_and_machine_health.sql

@@ -0,0 +1,32 @@
+-- GC pinning (#11)
+ALTER TABLE builds ADD COLUMN IF NOT EXISTS keep BOOLEAN NOT NULL DEFAULT false;
+ALTER TABLE jobsets ADD COLUMN IF NOT EXISTS keep_nr INTEGER NOT NULL DEFAULT 3;
+
+-- Recreate active_jobsets view to include keep_nr
+DROP VIEW IF EXISTS active_jobsets;
+CREATE VIEW active_jobsets AS
+SELECT
+    j.id,
+    j.project_id,
+    j.name,
+    j.nix_expression,
+    j.enabled,
+    j.flake_mode,
+    j.check_interval,
+    j.branch,
+    j.scheduling_shares,
+    j.created_at,
+    j.updated_at,
+    j.state,
+    j.last_checked_at,
+    j.keep_nr,
+    p.name as project_name,
+    p.repository_url
+FROM jobsets j
+JOIN projects p ON j.project_id = p.id
+WHERE j.state IN ('enabled', 'one_shot', 'one_at_a_time');
+
+-- Machine health tracking (#5)
+ALTER TABLE remote_builders ADD COLUMN IF NOT EXISTS consecutive_failures INTEGER NOT NULL DEFAULT 0;
+ALTER TABLE remote_builders ADD COLUMN IF NOT EXISTS disabled_until TIMESTAMP WITH TIME ZONE;
+ALTER TABLE remote_builders ADD COLUMN IF NOT EXISTS last_failure TIMESTAMP WITH TIME ZONE;