Adopt rustfmt and clean up clippy warnings

Formats the whole backend to rustfmt defaults and fixes all 23
outstanding clippy warnings (collapsible ifs, clone-to-slice-ref,
type-complexity, single-arm match, unnecessary is_none check), then
wires cargo fmt --check and -D warnings into make lint so both stay
enforced going forward.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TeyhRfMYjMditek6geDRoQ

tickets/094-adopt-rustfmt.md

@@ -1,34 +0,0 @@
-# 094 — Adopt rustfmt and format the codebase
-
-## Summary
-
-The Rust backend has no `rustfmt.toml` and isn't currently formatted
-to rustfmt's defaults: `cargo fmt -- --check` exits non-zero, reporting
-diffs across 24 files (352 diff hunks total).
-
-## Where
-
-- No `rustfmt.toml` / `.rustfmt.toml` exists at the repo root.
-- `cargo fmt -- --check` reports diffs in `src/api/{dto,mcp_server,mod}.rs`,
-  `src/cli/{mod,sessions}.rs`, `src/config/{mod,profile}.rs`, and most
-  of `src/engine/**` (`alerts.rs`, `data_value.rs`, `filter_datetime.rs`,
-  `filter_parser.rs`, `ingest.rs`, `logfmt.rs`, `message.rs`, `mod.rs`,
-  `promql.rs`, `rate.rs`, `snapshot/{filter,metrics,mod,params,query,sort}.rs`,
-  `tests.rs`). Heaviest: `src/engine/snapshot/mod.rs` (~65 hunks),
-  `src/config/profile.rs` (~38), `src/api/mod.rs` (~35),
-  `src/engine/alerts.rs` (~28).
-- `Makefile:26` (`lint-be`) currently only runs `cargo clippy
-  --all-targets` — no `cargo fmt --check` step.
-
-## Scope
-
-- Add a `rustfmt.toml` if any defaults should be overridden (otherwise
-  an empty/absent file is fine and rustfmt defaults apply — decide
-  whether to keep the file empty for documentation purposes or omit
-  it).
-- Run `cargo fmt` across the whole `src/` tree as a single,
-  non-functional formatting commit (don't mix with logic changes, to
-  keep the diff reviewable and `git blame`-friendly).
-- Add a `cargo fmt --check` step to `make lint` (or a new `make
-  fmt-check` wired into CI) so future changes can't drift out of
-  format again.

tickets/095-enforce-clippy-clean.md

@@ -1,41 +0,0 @@
-# 095 — Resolve clippy warnings and make lint enforce a clean run
-
-## Summary
-
-`make lint`'s backend target runs `cargo clippy --all-targets` with no
-`-D warnings` (or equivalent), so clippy warnings accumulate silently
-instead of failing the lint step. There are currently 23 unique
-warnings across 5 lint categories.
-
-## Where
-
-- `Makefile:26` (`lint-be:`) — `cargo clippy --all-targets`, no denial
-  flag.
-- `cargo clippy --all-targets` currently reports, by lint:
-  - `clippy::cloned_ref_to_slice_refs` — 10 occurrences, mostly in
-    `src/engine/alerts.rs` test code (e.g. around lines 469, 472, 493,
-    534).
-  - `clippy::collapsible_if` — 8 occurrences, concentrated in
-    `src/engine/snapshot/mod.rs` (lines 608, 940, 961) and
-    `src/engine/alerts.rs` (around lines 510, 512).
-  - `clippy::type_complexity` — 3 occurrences.
-  - `clippy::single_match` — 1 occurrence.
-  - `clippy::unnecessary_get_then_check` — 1 occurrence,
-    `src/engine/tests.rs:2234`
-    (`entries[1].params.get("level").is_none()`).
-
-## Scope
-
-- Fix all 23 current warnings (mechanical for most: `from_ref` for the
-  slice-ref clone cases, collapsing nested `if`s, `is_none()` →
-  `!contains_key`/`get().is_none()` equivalent simplification per
-  clippy's suggestion, breaking up the complex types clippy flags or
-  adding a type alias, converting the single-arm `match` to `if let`).
-- Once clean, change `Makefile:26` to deny warnings (e.g. `cargo
-  clippy --all-targets -- -D warnings`) so the target fails on any
-  future regression instead of just printing them.
-- Don't add blanket `#[allow(...)]`s to silence these — fix the
-  underlying code per clippy's suggestion, matching the existing
-  `#[allow(dead_code)]` uses elsewhere in the codebase which are for
-  genuinely-unused-for-now items, not a precedent for suppressing
-  active lints.