Per-request Notion token passthrough for HTTP transport

[ksinder]

Description

In Streamable HTTP transport, the Notion token is currently fixed at startup (NOTION_TOKEN / OPENAPI_MCP_HEADERS), which locks one deployment to a single Notion integration. This came up from a self-hosting user who wants one deployed server to serve multiple Notion integrations by passing the key as a header.

This PR adds an opt-in mode where each MCP client supplies its own Notion integration token per connection.

What changed

• New --enable-token-passthrough flag and ENABLE_TOKEN_PASSTHROUGH=true env var (default off — existing single-token behavior is unchanged).
• Per-connection token resolution on the initialize request, in order:
  1. dedicated Notion-Token header (preferred; coexists with the server's own Authorization gateway auth),
  2. Authorization: Bearer ntn_… — only when the server's own bearer auth is disabled (--unsafe-disable-auth),
  3. the startup env token as fallback, so passthrough + a default integration can coexist.
• MCPProxy / initProxy now accept explicit headers; env resolution stays the default when none are passed. Each token is bound to its MCP session.

Security

• Token shape validated (ntn_ / legacy secret_ prefixes), so a tenant's Notion token can never be confused with the server's gateway secret.
• Tokens are never logged — only a redacted prefix (ntn_…(NN chars)).
• A present-but-malformed Notion-Token returns 401 instead of silently falling back.
• README documents the TLS / gateway-auth guidance for multi-tenant use.

How was this change tested?

• Automated test (unit, integration, etc.) — new token.test.ts (14 tests) covers token shape-checking and header resolution precedence; proxy.test.ts extended for the explicit-header override. Full suite: 96 passing. npm run build passes.
• Manual test (provide reproducible testing steps below)

🤖 Generated with Claude Code

[ksinder]

Reconciled with #320 (page-as-Markdown tools) to keep the stack conflict-free.

This branch is now stacked on ksinder-markdown-tools (#320) — base retargeted accordingly. Two coordinated changes so per-request tokens work with the new per-operation Notion-Version model:

• notionHeadersForToken() no longer sets Notion-Version; it returns only Authorization.
• The NOTION_TOKEN env path in proxy.ts likewise no longer hardcodes Notion-Version (this came from Add page-as-Markdown tools (retrieve + update) #320).

Notion-Version is now sourced per-operation from the OpenAPI spec by HttpClient (added in #320), so passthrough requests automatically use 2026-03-11 for the markdown endpoints and 2025-09-03 for everything else — without each connection having to know the version. Tests updated accordingly; full suite green (102 tests).

Merge order: land #320 first, then this. (If #320 is dropped, this needs a one-line revert to put Notion-Version back into notionHeadersForToken.)