[Draft] Add experimental MCP server

[Freika]

Draft status

Exploratory MVP for discussion. The endpoint and two tools are functional, but the initial product/security boundaries should be reviewed before merge.

Goal

Let an MCP client answer two useful questions from an authenticated user's own Dawarich data:

• What did I do / where did I go during a bounded time range?
• What is my latest recorded location?

Implementation

• add the official Ruby MCP SDK
• expose a stateless Streamable HTTP endpoint at /api/v1/mcp
• authenticate with the existing Dawarich API key via a bearer authorization header
• reuse ApiController authentication, pending-payment checks, API rate limiting, host authorization, and response metadata
• expose get_timeline using Timeline::DayAssembler, strict inclusive ISO date semantics, a 7-calendar-day maximum, and a 500-entry preflight cap enforced before assembly
• map timeline data through an MCP-specific minimal serializer with a closed JSON Schema contract and runtime output validation
• expose get_latest_location using the authenticated user's non-anomalous, plan-scoped points
• return structured MCP content plus JSON text fallback
• mark both tools read-only, non-destructive, idempotent, and closed-world
• document generic client configuration and a curl initialization smoke test

Security and deployment

• every request creates a user-scoped server context from the bearer API key
• both tools use existing user and plan-window scopes
• timeline requests are bounded to 7 local calendar days and 500 returned entries using bounded preflight queries
• preflight track counting shares the assembler's end-exclusive local-day semantics
• runtime result validation enforces each tool's advertised JSON Schema
• stateless mode retains no MCP session state and works with multi-process/multi-instance deployments
• SDK Host/Origin checks remain enabled and use the trusted APPLICATION_HOSTS allowlist
• the reused API key has broader Dawarich API authority; documentation explicitly limits setup to fully trusted clients
• no write tools, OAuth server, resources, prompts, sampling, or live notifications

Questions before merge

1. Is reusing the full-access Dawarich API key acceptable for the MVP, or should the feature wait for scoped/revocable MCP tokens?
2. Are get_timeline and get_latest_location the right first tools?
3. Should MCP remain available wherever the read API is available, or be feature/plan gated?
4. Should a follow-up add visits/places search before this is announced?

Verification

• MCP request specs: 16 examples, 0 failures
• npm tests: 4 tests, 0 failures
• changed-file RuboCop: no offenses
• bundle exec rails zeitwerk:check: passed
• git diff --check: passed
• Brakeman: no warning in new MCP files; repository baseline warnings unchanged
• Bundler Audit: only pre-existing base-lockfile advisories in loofah and rails-html-sanitizer
• full RSpec: 6,871 examples passed; one pre-existing archive-task factory collision failed in-suite and passed when rerun alone
• independent Rails, QA/security, and product reviews: all approved stable staged SHA a43d4a42f3b5ac20112dfe06389fdec7e9fbcd3332e219cc790feefa820858f8

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✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch feat/mcp-server-mvp

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