Dev AI config: env var overrides in config_reader, LM Studio via .env

config_reader.py now merges environment variables (AI_PROVIDER,
LMSTUDIO_BASE_URL, LMSTUDIO_API_KEY, LMSTUDIO_MODEL, OLLAMA_*,
ANTHROPIC_*) on top of the JSON config file, so the dev .env file
can pin the AI connection without writing to the shared config volume.

docker-compose.dev.yml loads features/doc-service/.env (gitignored)
into the doc-service container so the token is never committed.

.env.example updated with all supported override variables and comments.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

features/doc-service/.env.example

@@ -1,3 +1,21 @@
 DATABASE_URL=postgresql+asyncpg://postgres:password@db:5432/destroying_sap
 DATA_DIR=/data/documents
 CONFIG_PATH=/config/doc_service_config.json
+
+# Optional AI provider overrides — if set, these take precedence over
+# whatever is stored in doc_service_config.json.
+# Useful for pinning a dev environment to a specific local AI instance
+# without touching the shared config volume.
+#
+# AI_PROVIDER=lmstudio
+# LMSTUDIO_BASE_URL=http://host.docker.internal:1234/v1
+# LMSTUDIO_API_KEY=your-lm-studio-token
+# LMSTUDIO_MODEL=local-model
+#
+# AI_PROVIDER=ollama
+# OLLAMA_BASE_URL=http://host.docker.internal:11434/v1
+# OLLAMA_MODEL=llama3.2
+#
+# AI_PROVIDER=anthropic
+# ANTHROPIC_API_KEY=sk-ant-...
+# ANTHROPIC_MODEL=claude-haiku-4-5-20251001