feat: add configurable account selection strategies

Refactor account selection into a strategy pattern with three options:
- Sticky: cache-optimized, stays on same account until rate-limited
- Round-robin: load-balanced, rotates every request
- Hybrid (default): smart distribution using health scores, token buckets, and LRU

The hybrid strategy uses multiple signals for optimal account selection:
health tracking for reliability, client-side token buckets for rate limiting,
and LRU freshness to prefer rested accounts.

Includes WebUI settings for strategy selection and unit tests.

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

src/account-manager/strategies/index.js

@@ -0,0 +1,85 @@
+/**
+ * Strategy Factory
+ *
+ * Creates and exports account selection strategy instances.
+ */
+
+import { StickyStrategy } from './sticky-strategy.js';
+import { RoundRobinStrategy } from './round-robin-strategy.js';
+import { HybridStrategy } from './hybrid-strategy.js';
+import { logger } from '../../utils/logger.js';
+import {
+    SELECTION_STRATEGIES,
+    DEFAULT_SELECTION_STRATEGY
+} from '../../constants.js';
+
+// Re-export strategy constants for convenience
+export const STRATEGY_NAMES = SELECTION_STRATEGIES;
+export const DEFAULT_STRATEGY = DEFAULT_SELECTION_STRATEGY;
+
+// Strategy display labels
+export const STRATEGY_LABELS = {
+    'sticky': 'Sticky (Cache Optimized)',
+    'round-robin': 'Round Robin (Load Balanced)',
+    'hybrid': 'Hybrid (Smart Distribution)'
+};
+
+/**
+ * Create a strategy instance
+ * @param {string} strategyName - Name of the strategy ('sticky', 'round-robin', 'hybrid')
+ * @param {Object} config - Strategy configuration
+ * @returns {BaseStrategy} The strategy instance
+ */
+export function createStrategy(strategyName, config = {}) {
+    const name = (strategyName || DEFAULT_STRATEGY).toLowerCase();
+
+    switch (name) {
+        case 'sticky':
+            logger.debug('[Strategy] Creating StickyStrategy');
+            return new StickyStrategy(config);
+
+        case 'round-robin':
+        case 'roundrobin':
+            logger.debug('[Strategy] Creating RoundRobinStrategy');
+            return new RoundRobinStrategy(config);
+
+        case 'hybrid':
+            logger.debug('[Strategy] Creating HybridStrategy');
+            return new HybridStrategy(config);
+
+        default:
+            logger.warn(`[Strategy] Unknown strategy "${strategyName}", falling back to ${DEFAULT_STRATEGY}`);
+            return new HybridStrategy(config);
+    }
+}
+
+/**
+ * Check if a strategy name is valid
+ * @param {string} name - Strategy name to check
+ * @returns {boolean} True if valid
+ */
+export function isValidStrategy(name) {
+    if (!name) return false;
+    const lower = name.toLowerCase();
+    return STRATEGY_NAMES.includes(lower) || lower === 'roundrobin';
+}
+
+/**
+ * Get the display label for a strategy
+ * @param {string} name - Strategy name
+ * @returns {string} Display label
+ */
+export function getStrategyLabel(name) {
+    const lower = (name || DEFAULT_STRATEGY).toLowerCase();
+    if (lower === 'roundrobin') return STRATEGY_LABELS['round-robin'];
+    return STRATEGY_LABELS[lower] || STRATEGY_LABELS[DEFAULT_STRATEGY];
+}
+
+// Re-export strategies for direct use
+export { StickyStrategy } from './sticky-strategy.js';
+export { RoundRobinStrategy } from './round-robin-strategy.js';
+export { HybridStrategy } from './hybrid-strategy.js';
+export { BaseStrategy } from './base-strategy.js';
+
+// Re-export trackers
+export { HealthTracker, TokenBucketTracker } from './trackers/index.js';