docs: state that pricing is also tiered by request input length

[argszero]

Summary

The transparent-billing line in the feature lists names the price modifiers a
model row can carry, and it named one of the two: peak-hour pricing.

Tiered pricing has shipped since the catalogue gained [[models.tiers]], and
billing::select_tier settles each request at the tier its input length
selects. No user-facing carrier mentioned it, so a reader could not learn from
the docs that a long request is billed at a different rate than a short one.

Four carriers describe that same roster and were short by the same entry, so
they move together:

Carrier	Before	After
README.md feature list	peak-hour pricing	peak-hour pricing and per-request input-length tiered pricing
README.en.md feature list	"with peak-hour pricing support"	"with peak-hour pricing and per-request input-length tiered pricing"
docs/user-stories.md capability list	peak-hour pricing	peak-hour pricing and per-request input-length tiered pricing
docs/plan-api-matrix.md §3 (price modifiers)	"含高峰时段价"	"含高峰时段价与分档价"
docs/plan-api-matrix.md §4 (model fields)	"缓存价与高峰价字段"	"缓存价、高峰价与分档价字段"

Changes

• Four documentation carriers now state that pricing is tiered by request input length
• Deliberately no counts and no model names — which rows carry a second tier block is owned by config/config.example.toml (via src/catalog_gate.rs), and a list copied into prose is the stale-copy family this repository keeps cleaning up
• The wording follows billing::select_tier's contract: the tier is chosen by that request's input length (input + cache-hit), not by a user's cumulative usage
• No configuration, behaviour or UI change — documentation only

Related Issue

None. This is a docs correction: the prose described a shipped capability
incompletely. docs/architecture.md already lists tiers_json correctly and
is left alone.

Tests

• cargo test — 579 passed, 0 failed
• cargo fmt --check — clean
• readme_gate run by name (both READMEs edited) — 4 passed; the two
  language versions still carry the same code spans, link targets, bare
  URLs and number literals
• No new unit test: this changes no code. The gates under src/ assert
  data parity and structure — catalog_gate checks the catalogue against
  config.example.toml, readme_gate checks that the two READMEs carry
  the same non-language-dependent facts — and none of them asserts the
  wording of a feature bullet, so there is no assertion to add here