http-auth done

[NEMEZIZ1234]

Summary

Adds HTTP API-key auth with per-key scope prefixes (multi-tenant): requests are rejected with 401 when auth is configured and a valid key isn't provided, and each key is restricted to its configured scope prefix. Enforced on both the MCP streamable-HTTP transport and the REST API.

Related issue

Closes #10

Changes

• src/mnema/auth.py (new) — AuthContext/Authenticator, ApiKeyAuthMiddleware (ASGI, for the MCP transport), parse_api_keys, and request_auth/set_auth/current_auth context-var helpers.
• config.py — new api_keys field (key1=user:alice:*,key2 — bare key = full access; empty = auth disabled), parsed via NoDecode + mode="before" validator (pydantic-settings ≥ 2.2).
• api/app.py — get_auth FastAPI dependency (401 when missing/invalid), ForbiddenError → 403 handler, /memories list clamped + denied for out-of-prefix scopes.
• server.py — _run_http wraps streamable_http_app() behind ApiKeyAuthMiddleware; FastMCP(host=...) so the DNS-rebinding allowlist matches remote hosts.
• service.py — _enforce_scope / _tenant_scope / _outside_prefix: writes → 403, reads clamp to the key's prefix, by-id get/update/forget → 404 on out-of-prefix ids, list_scopes/stats filtered, decay/reembed/summarize enforced.
• errors.py — added ForbiddenError.
• pyproject.toml — pydantic-settings>=2.2; version bumped to 0.5.0.
• Tests — new test_auth.py, test_http_auth.py (real MCP streamable client); extended test_api.py, test_config.py.
• Docs — README.md, docs/deployment.md, packages/mnema-extension/README.md (auth section); CHANGELOG [Unreleased] entry.

Checklist

• ruff check . is clean
• pytest passes (with the relevant [extra] installed if you touched a backend)
• New public functions / classes have docstrings
• Docs updated (README.md, SKILL.md, docs/) if user-facing behavior changed
• If this adds a backend or embedding provider: registered in the factory (__init__.py::make_*), added an [extra] in pyproject.toml, and added a test class

Notes for reviewer

• Auth is mandatory when configured (MNEMA_API_KEYS set) — 401 otherwise — and fully disabled when unset (backwards compatible).
• Env format is key=scope:prefix,key2 (comma-separated); pydantic-settings JSON-decodes complex env values before validators, so the field uses Annotated[..., NoDecode] (requires ≥ 2.2).
• Tenant semantics: explicit out-of-prefix writes → 403; reads on by-id ops return 404 (as MemoryNotFoundError) so tenants can't probe each other; list/search read paths clamp to the key's prefix when no scope is given.
• REST uses a FastAPI Depends; the context var needs no reset there (per-request ASGI task). The MCP transport needs a middleware (reset per stream) per the issue spec.
• FastMCP(host=cfg.http_host) was required so the loopback DNS-rebinding allowlist matches the clients served; a non-loopback host disables rebinding protection, which auth now covers.