docs: Add crate-level architecture documentation

[jan-auer]

This is the starting point for more extensive documentation. The architecture docs added here cover the high-level design of each crate. Over time, we will expand documentation into submodules and individual types, which may elaborate on the concepts introduced here and push some of this content further down in the hierarchy. Expect these top-level docs to evolve as that happens.

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Adds docs/architecture.md to each of the three main crates (objectstore-service, objectstore-server, objectstore-types) and wires them in as crate-level rustdoc via include_str!. This uses the same pattern as the Rust client crate's README and requires zero CI/infrastructure changes — the existing cargo doc build and GitHub Pages deploy pick these up automatically.

objectstore-service — the primary doc, covering the two-tier backend system (BigTable + GCS), the 1 MiB size threshold and streaming-based backend routing, redirect tombstones (what they solve, how each CRUD operation interacts with them), the Backend trait and its implementations, object identification (ObjectId/ObjectContext/scopes), and the streaming architecture.

objectstore-server — server-level concerns: endpoint routes, end-to-end request flow through middleware/extractors/auth/admission control, JWT authentication with EdDSA and key rotation, figment-based layered configuration, token-bucket rate limiting with EWMA bandwidth metering, and glob-based killswitches.

objectstore-types — the shared type vocabulary: Metadata fields and their HTTP header mapping (x-sn-* / x-snme-* prefixes), the scope system (validation rules, storage vs API path formats), expiration policies (Manual/TTL/TTI), compression, and permissions.

Also updates AGENTS.md with instructions to check architecture docs when making changes, and cleans up the binary crate's (main.rs) doc comment to point at the library crate instead of duplicating a stale description.

[github-actions]

Semver Impact of This PR

🟢 Patch (bug fixes)

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New Features ✨

• (types) Add origin as built-in metadata field by jan-auer in #292

Documentation 📚

• (clients) Restructure Rust and Python client READMEs by jan-auer in #294

• Add crate-level architecture documentation by jan-auer in #304

Internal Changes 🔧

• (types) Move tombstone + custom-time out of Metadata header methods by jan-auer in #303

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