Add anti-bot detection, retry, and fallback system

Automatically detect when crawls are blocked by anti-bot systems
(Akamai, Cloudflare, PerimeterX, DataDome, Imperva, etc.) and
escalate through configurable retry and fallback strategies.

New features on CrawlerRunConfig:
- max_retries: retry rounds when blocking is detected
- fallback_proxy_configs: list of fallback proxies tried each round
- fallback_fetch_function: async last-resort function returning raw HTML

New field on ProxyConfig:
- is_fallback: skip proxy on first attempt, activate only when blocked

Escalation chain per round: main proxy → fallback proxies in order.
After all rounds: fallback_fetch_function as last resort.

Detection uses tiered heuristics — structural HTML markers (high
confidence) trigger on any page, generic patterns only on short
error pages to avoid false positives.

docs/md_v2/api/parameters.md

@@ -109,8 +109,11 @@ We group them by category.
 | **`timezone_id`**      | `str or None` (None)      | Browser's timezone (e.g., "America/New_York", "Europe/Paris").                                         |
 | **`geolocation`**      | `GeolocationConfig or None` (None) | GPS coordinates configuration. Use `GeolocationConfig(latitude=..., longitude=..., accuracy=...)`. |
 | **`fetch_ssl_certificate`** | `bool` (False)       | If `True`, fetches and includes SSL certificate information in the result.                             |
-| **`proxy_config`**           | `ProxyConfig or dict or None` (None) | Proxy configuration for this specific crawl. Can override browser-level proxy settings.          |
+| **`proxy_config`**           | `ProxyConfig or dict or None` (None) | Proxy configuration for this specific crawl. Can override browser-level proxy settings. Set `is_fallback=True` on the ProxyConfig to only use the proxy when anti-bot blocking is detected. |
 | **`proxy_rotation_strategy`** | `ProxyRotationStrategy` (None)      | Strategy for rotating proxies during crawl operations.                                           |
+| **`max_retries`**            | `int` (0)                            | Number of retry rounds when anti-bot blocking is detected. Each round tries the main proxy and all fallback proxies. |
+| **`fallback_proxy_configs`** | `list[ProxyConfig]` ([])             | List of fallback proxies tried in order within each retry round after the main proxy fails.       |
+| **`fallback_fetch_function`**| `async (str) -> str or None` (None)  | Async function called as last resort after all retries are exhausted. Takes URL, returns raw HTML. See [Anti-Bot & Fallback](../advanced/anti-bot-and-fallback.md). |
 
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