refactor: consolidate stale logic, use transaction helper, add migrations

- Add StaleLevel enum (ok, WARN, STALE, DEAD) to types.nim
- Extract computeStaleLevel() as single source of truth
- Simplify isStale() and staleLevel() to use computeStaleLevel()
- Refactor transition() and transitionToFailed() to use withTransaction
- Add schema migration infrastructure:
  - CurrentSchemaVersion constant
  - Migrations sequence for incremental upgrades
  - getSchemaVersion() and runMigrations() procs
  - initSchema() now runs pending migrations

Closes: skills-dtk, skills-8fd, skills-9ny

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/worker/db.nim

@@ -92,13 +92,56 @@ CREATE TABLE IF NOT EXISTS export_state (
 );
 """
 
+const
+  CurrentSchemaVersion* = 1
+
+  # Migrations are applied in order. Each migration upgrades FROM the version
+  # number to the next version. Add new migrations at the end.
+  # Format: (fromVersion, sql)
+  Migrations*: seq[tuple[fromVersion: int, sql: string]] = @[
+    # Example migration (uncomment when needed):
+    # (1, "ALTER TABLE workers ADD COLUMN priority INTEGER DEFAULT 2"),
+  ]
+
+proc getSchemaVersion*(db: DbConn): int =
+  ## Get current schema version from database
+  let row = db.one("SELECT value FROM meta WHERE key = 'schema_version'")
+  if row.isNone:
+    return 0
+  try:
+    return parseInt(row.get[0].fromDbValue(string))
+  except ValueError:
+    return 0
+
+proc runMigrations*(db: DbConn) =
+  ## Apply any pending schema migrations
+  let currentVersion = db.getSchemaVersion()
+
+  for migration in Migrations:
+    if migration.fromVersion >= currentVersion:
+      continue  # Skip already-applied migrations
+
+    # Run migration in a transaction
+    db.withTransaction:
+      db.execScript(migration.sql)
+      let newVersion = migration.fromVersion + 1
+      db.exec("UPDATE meta SET value = ? WHERE key = 'schema_version'",
+              $newVersion)
+
+    logWarn("runMigrations", "Applied migration " & $migration.fromVersion &
+            " → " & $(migration.fromVersion + 1))
+
 proc initSchema*(db: DbConn) =
-  ## Initialize database schema
+  ## Initialize database schema and run migrations
   db.execScript(Schema)
   # Insert meta if not exists
-  db.exec("INSERT OR IGNORE INTO meta (key, value) VALUES ('schema_version', '1')")
+  db.exec("INSERT OR IGNORE INTO meta (key, value) VALUES ('schema_version', ?)",
+          $CurrentSchemaVersion)
   db.exec("INSERT OR IGNORE INTO export_state (id, last_seq) VALUES (1, 0)")
 
+  # Run any pending migrations for existing databases
+  runMigrations(db)
+
 proc openBusDb*(dbPath: string = BusDbPath): DbConn =
   ## Open database with required PRAGMAs. One connection per thread.
   try:

src/worker/state.nim

@@ -51,8 +51,7 @@ proc transition*(db: DbConn, taskId: string, fromState, toState: WorkerState) =
   if not canTransition(fromState, toState):
     raise newException(InvalidTransition, &"Invalid transition: {fromState} → {toState}")
 
-  db.exec("BEGIN IMMEDIATE")
-  try:
+  db.withTransaction:
     let row = db.one(
       "SELECT state FROM workers WHERE task_id = ?",
       taskId
@@ -83,15 +82,9 @@ proc transition*(db: DbConn, taskId: string, fromState, toState: WorkerState) =
         "task_id": taskId
       })
 
-    db.exec("COMMIT")
-  except CatchableError:
-    db.exec("ROLLBACK")
-    raise
-
 proc transitionToFailed*(db: DbConn, taskId: string, reason: string = "") =
   ## Transition any state to FAILED
-  db.exec("BEGIN IMMEDIATE")
-  try:
+  db.withTransaction:
     let row = db.one(
       "SELECT state FROM workers WHERE task_id = ?",
       taskId
@@ -121,11 +114,6 @@ proc transitionToFailed*(db: DbConn, taskId: string, reason: string = "") =
         "reason": reason
       })
 
-    db.exec("COMMIT")
-  except CatchableError:
-    db.exec("ROLLBACK")
-    raise
-
 proc getState*(db: DbConn, taskId: string): Option[WorkerState] =
   ## Get current state for a worker
   let row = db.one(
@@ -195,31 +183,29 @@ proc createWorker*(db: DbConn, taskId, branch, worktree: string,
     stateChangedAt: msToTime(tsMs),
   )
 
-proc isStale*(info: WorkerInfo): bool =
-  ## Check if worker is stale based on heartbeat age
+proc computeStaleLevel*(info: WorkerInfo): StaleLevel =
+  ## Compute staleness level based on heartbeat age.
+  ## Only ASSIGNED and WORKING states can be stale.
   if info.state notin {wsAssigned, wsWorking}:
-    return false
+    return slOk
 
   if info.lastHeartbeat == Time():
-    return true  # No heartbeat yet
+    return slDead  # No heartbeat yet
 
-  let age = (epochMs() - toUnix(info.lastHeartbeat) * 1000)
-  return age > StaleThresholdMs
+  let age = epochMs() - toUnix(info.lastHeartbeat) * 1000
+  if age > DeadThresholdMs:
+    slDead
+  elif age > StaleThresholdMs:
+    slStale
+  elif age > StaleWarnThresholdMs:
+    slWarn
+  else:
+    slOk
+
+proc isStale*(info: WorkerInfo): bool =
+  ## Check if worker is stale (STALE or DEAD level)
+  info.computeStaleLevel in {slStale, slDead}
 
 proc staleLevel*(info: WorkerInfo): string =
-  ## Get stale level: ok, WARN, STALE, DEAD
-  if info.state notin {wsAssigned, wsWorking}:
-    return "ok"
-
-  if info.lastHeartbeat == Time():
-    return "DEAD"
-
-  let age = (epochMs() - toUnix(info.lastHeartbeat) * 1000)
-  if age > DeadThresholdMs:
-    return "DEAD"
-  elif age > StaleThresholdMs:
-    return "STALE"
-  elif age > StaleWarnThresholdMs:
-    return "WARN"
-  else:
-    return "ok"
+  ## Get stale level as string: ok, WARN, STALE, DEAD
+  $info.computeStaleLevel

src/worker/types.nim

@@ -41,6 +41,13 @@ type
     hsWorking = "working"
     hsBlocked = "blocked"
 
+  StaleLevel* = enum
+    ## Staleness level based on heartbeat age
+    slOk = "ok"
+    slWarn = "WARN"
+    slStale = "STALE"
+    slDead = "DEAD"
+
   # Errors
   InvalidTransition* = object of CatchableError
   StaleState* = object of CatchableError