Add SaaS AI engine (#5907)

engine/config/.env.example

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+# Claude API (Anthropic)
+STIRLING_ANTHROPIC_API_KEY=
+
+# OpenAI API (Optional - for GPT models)
+STIRLING_OPENAI_API_KEY=
+STIRLING_OPENAI_BASE_URL=
+
+# Model Configuration
+STIRLING_SMART_MODEL=claude-sonnet-4-5-20250929
+STIRLING_FAST_MODEL=claude-haiku-4-5-20251001
+
+# GPT-5 Reasoning Settings (only used if SMART_MODEL is gpt-5)
+# Options: minimal, low, medium, high, xhigh
+STIRLING_SMART_MODEL_REASONING_EFFORT=medium
+STIRLING_FAST_MODEL_REASONING_EFFORT=minimal
+
+# GPT-5 Text Verbosity (only used if SMART_MODEL is gpt-5)
+# Options: minimal, low, medium, high
+STIRLING_SMART_MODEL_TEXT_VERBOSITY=medium
+STIRLING_FAST_MODEL_TEXT_VERBOSITY=low
+
+# Output token limits. STIRLING_AI_MAX_TOKENS is a global override (leave empty to use per-model defaults)
+STIRLING_AI_MAX_TOKENS=
+STIRLING_SMART_MODEL_MAX_TOKENS=8192
+STIRLING_FAST_MODEL_MAX_TOKENS=2048
+STIRLING_CLAUDE_MAX_TOKENS=4096
+STIRLING_DEFAULT_MODEL_MAX_TOKENS=4096
+
+# PostHog Analytics
+STIRLING_POSTHOG_API_KEY=
+STIRLING_POSTHOG_HOST=https://eu.i.posthog.com
+
+# Backend Configuration
+STIRLING_JAVA_BACKEND_URL=http://localhost:8080
+STIRLING_JAVA_BACKEND_API_KEY=123456789
+STIRLING_JAVA_REQUEST_TIMEOUT_SECONDS=30
+
+# Logging and Debugging
+STIRLING_AI_RAW_DEBUG=0
+STIRLING_FLASK_DEBUG=0
+# Override the log file directory (defaults to ./logs in server mode, OS log dir in desktop mode)
+STIRLING_LOG_PATH=
+# Enable verbose table detection debug logging in the PDF text editor
+STIRLING_PDF_EDITOR_TABLE_DEBUG=0
+# Set to true when running as a Tauri desktop app (switches to OS-native log directory)
+STIRLING_PDF_TAURI_MODE=false
+
+# AI Settings
+STIRLING_AI_STREAMING=true
+STIRLING_AI_PREVIEW_MAX_INFLIGHT=3
+STIRLING_AI_REQUEST_TIMEOUT=70