feat(embedder): configurable request timeout and max retries

[kryptt]

Problem

The HTTP client timeout (60s) and retry max-attempts (5) used by the embedders are compiled-in constants. They're well-tuned for OpenAI's hosted API but break down against slow self-hosted endpoints — a shared `ollama-cuda` instance handling other inference jobs can easily take > 60s to return a single batch's embeddings, especially under model warm-up or memory pressure.

Symptom I hit while building a large workspace through a self-hosted Ollama:

```
Warning: failed to initialize runtime for emacs-source: initial indexing failed:
failed to embed batches: failed to send request to OpenAI:
Post "https://ollama.hr-home.xyz/v1/embeddings\":
context deadline exceeded (Client.Timeout exceeded while awaiting headers)
```

The watcher abandons the whole project on the first batch that exceeds 60s, even though the underlying queue is still making forward progress on the server side. Larger projects (in my case ~47k chunks across emacs-source) become un-indexable end-to-end.

Solution

Two new optional fields on `EmbedderConfig`:

```yaml
embedder:
provider: openai
request_timeout_seconds: 600 # default: 0 = preserve historical 60s
max_retries: 8 # default: 0 = preserve historical 5
```

• `request_timeout_seconds` becomes the `http.Client.Timeout` for the chosen embedder. Each of the five embedders (openai / ollama / lmstudio / openrouter / synthetic) gets a `With…Timeout` option; the factory threads the value through. Synthetic's existing 90s default is preserved when the field is unset.
• `max_retries` is honored by providers that already implement retry today — only `openai` does, so the field's docstring calls this out. Other providers silently ignore it (no behavior change). For openai, the value caps `RetryPolicy.MaxAttempts`.

Both fields are zero-valued by default and *explicitly preserve existing behavior* in that case. A bare `embedder:` block produces the same embedder as before this PR.

Test plan

Two new tests in `embedder/factory_test.go`:

Test	Asserts
`TestNewFromConfig_RequestTimeoutAndMaxRetries`	`request_timeout_seconds: 300` results in 5-minute `client.Timeout`; `max_retries: 9` results in `retryPolicy.MaxAttempts == 9`
`TestNewFromConfig_TimeoutDefaultsPreserved`	with both fields unset, defaults stay at 60s / 5 attempts

All existing tests continue to pass (`go test ./...`).