feat(ui): add structured JSON logging to all server-side routes and lib

Introduces a logger.ts module emitting slog-compatible JSON lines to stderr.
Replaces silent catch blocks and console.error calls throughout minio.ts,
pocketbase.ts, hooks.server.ts, login, books, browse, and all API routes so
auth/registration failures, MinIO presign errors, and scraper proxy failures
are now visible in container logs.

ui/src/lib/server/logger.ts

@@ -0,0 +1,37 @@
+/**
+ * Structured server-side logger.
+ *
+ * Emits JSON lines to stderr so they appear in container/process logs without
+ * polluting stdout (which Node's HTTP layer uses for responses).
+ *
+ * Format mirrors Go's log/slog default JSON output:
+ *   {"time":"…","level":"ERROR","msg":"…","context":"pocketbase",...extra}
+ *
+ * Usage:
+ *   import { log } from '$lib/server/logger';
+ *   log.error('pocketbase', 'auth failed', { status: 401, url });
+ *   log.warn('minio', 'presign slow', { slug, n, ms: elapsed });
+ *   log.info('auth', 'user registered', { username });
+ */
+
+type Level = 'DEBUG' | 'INFO' | 'WARN' | 'ERROR';
+type Extra = Record<string, unknown>;
+
+function emit(level: Level, context: string, msg: string, extra?: Extra): void {
+	const entry: Record<string, unknown> = {
+		time: new Date().toISOString(),
+		level,
+		context,
+		msg,
+		...extra
+	};
+	// Write to stderr — never stdout
+	process.stderr.write(JSON.stringify(entry) + '\n');
+}
+
+export const log = {
+	debug: (context: string, msg: string, extra?: Extra) => emit('DEBUG', context, msg, extra),
+	info:  (context: string, msg: string, extra?: Extra) => emit('INFO',  context, msg, extra),
+	warn:  (context: string, msg: string, extra?: Extra) => emit('WARN',  context, msg, extra),
+	error: (context: string, msg: string, extra?: Extra) => emit('ERROR', context, msg, extra),
+};