Drop contexts resource; bump runtime; drop direct nvisy-schema dep

[martsokha]

Summary

Upstream runtime dropped the nvisy-context crate and the contexts parameter from Engine::analyze_document, leaving the server's contexts resource with no backing engine support. This PR removes that resource end to end, reorders the pipeline/policy migrations, bumps the runtime dependency, and drops the now-redundant direct nvisy-schema dependency.

Changes

Remove the contexts resource

• DB: drop the workspace_contexts table and the workspace_pipeline_contexts join; delete their model, query, and constraint modules; regenerate schema.rs (zero context-table references remain).
• Server: delete the contexts handler, request/response DTOs, and routes; remove the Contexts builtin module, the ViewContexts/ManageContexts permissions, the OpenAPI Contexts tag, and all pipeline/run context references (context_slugs, resolve_contexts, the analyze_document contexts argument).

Reorder migrations

• Pipelines (045014) now run before policies (045015).
• The workspace_pipeline_policies join moves into the policies migration, where both parent tables (workspace_pipelines, workspace_policies) exist.
• The full up/down chain applies and reverts cleanly (verified against a live DB).

Runtime + dependency

• Bump the runtime git dep to 4e8f32d4.
• Drop the direct nvisy-schema dependency — nvisy-engine re-exports file/plan/policy, so the server consumes those types through nvisy_engine. nvisy-schema remains only as a transitive dep of the engine.

Verification

• cargo check --all-features --workspace ✅
• cargo clippy --all-targets --all-features --workspace -- -D warnings ✅
• cargo +nightly fmt --all -- --check ✅
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --all-features --workspace ✅
• cargo test --all-features --workspace ✅ (all suites pass)
• Migrations regenerated; full up/down chain verified against a live Postgres.

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