feat(server): pluggable AI providers - any open-standard or commercial AI

Lets the owner point transcription and summarisation at any AI: an open-standard
endpoint (OpenAI-compatible / local faster-whisper / Ollama) or a commercial API
(OpenAI, Anthropic, Gemini). Config-driven, self-hostable, no lock-in.

What changed:
- server/app/providers/: provider layer.
  - base.py: Transcriber/Summariser protocols + shared summary prompt + tolerant JSON
    parser (uniform Summary shape across providers).
  - summary.py: OpenAICompatibleSummariser (any /chat/completions - OpenAI, Groq,
    OpenRouter, LocalAI, LM Studio, vLLM, Ollama /v1) and AnthropicSummariser (Claude
    via the official anthropic SDK; Messages API has no OpenAI-compatible endpoint).
  - transcription.py: OpenAICompatibleTranscriber (/audio/transcriptions - OpenAI,
    Groq, self-hosted whisper server) and LocalWhisperTranscriber (faster-whisper,
    execution wired in M5).
  - factory.py: builds the configured providers with per-provider defaults
    (anthropic -> claude-opus-4-8, openai_compatible -> gpt-4o-mini, ollama -> llama3.1).
- config.py + .env.example: transcription_provider / llm_provider selectors + base_url,
  key, model settings; local faster-whisper and Ollama kept as the self-hosted defaults.
- main.py: /health now reports the resolved provider names (no secrets).
- requirements.txt: httpx drives all HTTP providers; anthropic + faster-whisper are
  optional, only for their respective providers.
- docs/ai-providers.md: config recipes for OpenAI, Groq, Anthropic, Gemini, LocalAI,
  LM Studio, Ollama, self-hosted whisper.
- state/: DECISIONS, ARCHITECTURE, TODO updated.

Why:
- The user asked to connect the device to any open standard AI or commercial one; this
  is also the core differentiator vs Plaud's locked cloud.

Notes:
- Anthropic provider uses the official SDK and defaults to claude-opus-4-8 (per the
  claude-api guidance). AI deps are optional per chosen provider. Modules byte-compile
  cleanly; end-to-end wiring into the ingest pipeline lands with M5.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/app/providers/factory.py

@@ -0,0 +1,40 @@
+# SPDX-License-Identifier: GPL-3.0-only
+"""Build the configured providers from settings, applying sensible per-provider defaults."""
+from __future__ import annotations
+
+from ..config import settings
+from .base import Summariser, Transcriber
+from .summary import AnthropicSummariser, OpenAICompatibleSummariser
+from .transcription import LocalWhisperTranscriber, OpenAICompatibleTranscriber
+
+
+def build_summariser() -> Summariser:
+    provider = settings.llm_provider
+    model = settings.llm_model
+
+    if provider == "anthropic":
+        return AnthropicSummariser(settings.llm_api_key, model or "claude-opus-4-8")
+
+    if provider == "openai_compatible":
+        return OpenAICompatibleSummariser(
+            settings.llm_base_url, settings.llm_api_key, model or "gpt-4o-mini"
+        )
+
+    # Default: Ollama via its OpenAI-compatible /v1 endpoint (fully self-hosted).
+    base = settings.ollama_url.rstrip("/") + "/v1"
+    return OpenAICompatibleSummariser(
+        base, settings.llm_api_key or "ollama", model or settings.ollama_model or "llama3.1"
+    )
+
+
+def build_transcriber() -> Transcriber:
+    if settings.transcription_provider == "openai_compatible":
+        return OpenAICompatibleTranscriber(
+            settings.transcription_base_url,
+            settings.transcription_api_key,
+            settings.transcription_model or "whisper-1",
+        )
+    # Default: local faster-whisper (self-hosted).
+    return LocalWhisperTranscriber(
+        settings.whisper_model, settings.whisper_device, settings.whisper_compute_type
+    )