docs: document pricing diagnostics, metrics endpoint, and DiffOutcome per-1k token fields

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Summary

Closes documentation gaps introduced by v1.0.4 (metrics endpoint + pricing diagnostics) that were not fully reflected in the reference docs.

Docs added/updated

docs/operations-and-policy.md

• Expanded the Pricing and model change detection section to cover the six new DiffOutcome per-1k token rate fields added in v1.0.4 (baseline_input_usd_per_1k_tokens, baseline_output_usd_per_1k_tokens, baseline_cached_input_usd_per_1k_tokens, and their candidate counterparts).
• Documents the CLI Per-1k token prices output line (conditionally printed when all four input/output rates are present).
• Documents the pricing.prices object in the POST /v1/diff HTTP response (always present; fields null when rate not set in the table).
• Cross-references the web UI banner behaviour and links to web-ui.md.

docs/sdk.md

• Added a GET /v1/metrics (no SDK wrapper) subsection explaining that the endpoint has no dedicated method in FlightdeckClient/AsyncFlightdeckClient, with a copy-pasteable httpx example and a link to the HTTP API reference.

docs/web-ui.md

• Updated the DiffPage pricing change warning bullet to mention the new per-1k input/output price delta line shown in the fd-alert--warn banner (rendered when all four rates are non-null; toFixed(6) formatting).

Codepaths covered

• src/flightdeck/operations.py — DiffOutcome per-1k rate fields
• src/flightdeck/server/routes/actions.py — pricing.prices in POST /v1/diff response
• src/flightdeck/server/routes/metrics.py — GET /v1/metrics
• src/flightdeck/storage.py — get_ledger_counters()
• src/flightdeck/cli/main.py — Per-1k token prices CLI output
• web/src/pages/DiffPage.tsx — pickPrices() and banner rendering

Key knowledge gaps addressed

• DiffOutcome carries six per-1k token rate fields since v1.0.4 — these were undocumented outside the source code and CHANGELOG.md.
• GET /v1/metrics had no guidance on how to consume it from Python (no SDK wrapper exists).
• The web UI's pricing banner now shows token price deltas, but web-ui.md only described the provider/version/model display from v1.0.3.

Verification

• uv run python -m ruff check src tests — clean
• uv run python -m pytest — 94 passed