feat: D3 foundation — retry schema, shared types, health client, backoff helper

- Add retry_count/next_retry_at to jobs table via guarded idempotent ALTER TABLE
  (PRAGMA table_info existence check, try/catch for safety on restart)
- Add retryCount/nextRetryAt to Job type, rowToJob(), updateJob(), resetJob()
- Add getHealth() returning structured WhisperHealth (status, gpu_name,
  vram_total_mb, model, queue_depth, model_state) distinct from
  existing checkHealth() boolean helper
- Create retryWithBackoff() pure helper in src/lib/server/retry.ts with
  exponential backoff + jitter and onAttempt callback — no persistence owned
- Tests: retry column read/write/reset, getHealth() smoke tests,
  retryWithBackoff() edge cases (success, failure, backoff timing, onAttempt)

src/lib/server/db.ts

@@ -4,6 +4,18 @@ import { existsSync, mkdirSync } from 'fs';
 import { join } from 'path';
 import type { Job, JobStatus, AudioMode, PushSubscription } from '$lib/types.js';
 
+/**
+ * Add a column to a table if it does not already exist.
+ * Uses PRAGMA table_info for an existence check, consistent with the
+ * existing CREATE TABLE IF NOT EXISTS idiom — no migration tool introduced.
+ */
+function addColumnIfMissing(table: string, column: string, typeDef: string): void {
+	const columns = db.pragma(`table_info(${table})`) as { name: string }[];
+	if (!columns.some((c) => c.name === column)) {
+		db.exec(`ALTER TABLE ${table} ADD COLUMN ${column} ${typeDef}`);
+	}
+}
+
 const DATA_DIR = process.env.DATA_DIR ?? join(process.env.HOME ?? '/tmp', '.whisper-pwa');
 if (!existsSync(DATA_DIR)) mkdirSync(DATA_DIR, { recursive: true });
 
@@ -37,6 +49,14 @@ db.exec(`
 	);
 `);
 
+// Add retry columns idempotently (may already exist on restart)
+try {
+	addColumnIfMissing('jobs', 'retry_count', 'INTEGER DEFAULT 0');
+	addColumnIfMissing('jobs', 'next_retry_at', 'TEXT');
+} catch (e) {
+	console.warn('[db] failed to add retry columns:', e);
+}
+
 const stmts = {
 	insertJob: db.prepare(`
 		INSERT INTO jobs (id, status, title, source, audio_mode)
@@ -52,6 +72,8 @@ const stmts = {
 			output_dir    = @outputDir,
 			segments_json = @segmentsJson,
 			error         = @error,
+			retry_count   = @retryCount,
+			next_retry_at = @nextRetryAt,
 			updated_at    = datetime('now')
 		WHERE id = @id
 	`),
@@ -64,6 +86,7 @@ const stmts = {
 			status = 'pending', progress = 0, error = NULL,
 			mean_volume = NULL, whisper_job_id = NULL,
 			output_dir = NULL, segments_json = NULL,
+			retry_count = 0, next_retry_at = NULL,
 			updated_at = datetime('now')
 		WHERE id = @id
 	`),
@@ -92,6 +115,8 @@ function rowToJob(row: Record<string, unknown>): Job {
 		outputDir: row.output_dir as string | null,
 		segmentsJson: row.segments_json as string | null,
 		error: row.error as string | null,
+		retryCount: row.retry_count as number | null,
+		nextRetryAt: row.next_retry_at as string | null,
 		createdAt: row.created_at as string,
 		updatedAt: row.updated_at as string
 	};
@@ -125,7 +150,9 @@ export function updateJob(job: Partial<Job> & { id: string }): void {
 		progress: merged.progress,
 		outputDir: merged.outputDir,
 		segmentsJson: merged.segmentsJson,
-		error: merged.error
+		error: merged.error,
+		retryCount: merged.retryCount ?? null,
+		nextRetryAt: merged.nextRetryAt ?? null
 	});
 }