refactor(settings): split Settings into per-domain modules

docsgpt/core/settings.py had grown to 258 fields in one 600-line class,
touched by about two commits a week, with related settings scattered
(GitHub ingest caps inside the embeddings block, API keys in four places,
the OpenAI Responses knobs 100 lines from the other OpenAI fields).

It is now a package: one module per domain (auth, llm, embeddings,
retrieval, vectorstores, database, workers, ingestion, ocr, storage,
connectors, server, events, agents, guardrails, scheduler, sandbox,
speech), each a SettingsGroup owning its fields and validators, composed
by multiple inheritance into the same flat Settings class. Every
attribute name, type, default, alias and constraint is unchanged, so
settings.NAME reads, .env files and test monkeypatches all keep working;
the import path docsgpt.core.settings is the package. Settings.normalize_api_key
is kept as a classmethod for callers that reuse it.

The comment above or beside each field became its Field(description=...),
so the definitions are visible to tooling; the next commit generates the
docs reference from them.

Pitfall recorded for future groups: pydantic collects validators by
method name across the MRO, so two groups naming a validator the same
would silently keep only one. Each group's validator has a unique name.

docs/content/Guides/Architecture.mdx

@@ -251,4 +251,4 @@ The main extension points in [arc53/DocsGPT](https://github.com/arc53/DocsGPT) a
 | Parsing and workers | [`docsgpt/parser/`](https://github.com/arc53/DocsGPT/tree/main/docsgpt/parser), [`docsgpt/worker.py`](https://github.com/arc53/DocsGPT/blob/main/docsgpt/worker.py), [`docsgpt/api/user/tasks.py`](https://github.com/arc53/DocsGPT/blob/main/docsgpt/api/user/tasks.py) |
 | Models and vector stores | [`docsgpt/llm/`](https://github.com/arc53/DocsGPT/tree/main/docsgpt/llm), [`docsgpt/vectorstore/`](https://github.com/arc53/DocsGPT/tree/main/docsgpt/vectorstore) |
 | Storage and events | [`docsgpt/storage/`](https://github.com/arc53/DocsGPT/tree/main/docsgpt/storage), [`docsgpt/streaming/`](https://github.com/arc53/DocsGPT/tree/main/docsgpt/streaming), [`docsgpt/events/`](https://github.com/arc53/DocsGPT/tree/main/docsgpt/events) |
-| Configuration, UI, and deployment | [`docsgpt/core/settings.py`](https://github.com/arc53/DocsGPT/blob/main/docsgpt/core/settings.py), [`frontend/`](https://github.com/arc53/DocsGPT/tree/main/frontend), [`deployment/`](https://github.com/arc53/DocsGPT/tree/main/deployment) |
+| Configuration, UI, and deployment | [`docsgpt/core/settings/`](https://github.com/arc53/DocsGPT/tree/main/docsgpt/core/settings), [`frontend/`](https://github.com/arc53/DocsGPT/tree/main/frontend), [`deployment/`](https://github.com/arc53/DocsGPT/tree/main/deployment) |

docs/content/Guides/compression.md

@@ -19,7 +19,7 @@ The compression system operates on a "summarize and truncate" principle:
 
 ## Configuration
 
-You can configure the compression behavior in your `.env` file or `docsgpt/core/settings.py`:
+You can configure the compression behavior in your `.env` file or `docsgpt/core/settings/agents.py`:
 
 | Setting | Default | Description |
 | :--- | :--- | :--- |

docs/content/quickstart.mdx

@@ -124,6 +124,6 @@ To work from the source tree, for example to build the images yourself, use `set
 
 ## Advanced Configuration
 
-For more advanced customization of DocsGPT settings, such as configuring vector stores, embedding models, and other parameters, please refer to the [DocsGPT Settings documentation](/Deploying/DocsGPT-Settings). This guide explains how to modify the `.env` file or `settings.py` for deeper configuration.
+For more advanced customization of DocsGPT settings, such as configuring vector stores, embedding models, and other parameters, please refer to the [DocsGPT Settings documentation](/Deploying/DocsGPT-Settings). This guide explains how to configure DocsGPT through the `.env` file, and links to the full settings reference.
 
 Enjoy using DocsGPT!

docs/runbooks/sse-notifications.md

@@ -320,7 +320,7 @@ redis-cli -n 2 DEL user:<id>:stream
 
 ## Settings reference
 
-Everything in `docsgpt/core/settings.py`:
+Everything in `docsgpt/core/settings/events.py`:
 
 | Setting                                       | Default | Purpose                                       |
 | --------------------------------------------- | ------- | --------------------------------------------- |