chore(api): lift ROUTING_TARGETS to a leaf module (breaks import cycle)

[hizrianraz]

Pure hygiene; independent of any other in-flight PR. Per the GATE 4 GO brief: "ROUTING_TARGETS → constants module cleanup PR (retire the two noqa: PLC0415 lazy imports from AIN-285). Pure hygiene, independent, ship it."

Motivation

`services/capture_invariant.py` pulled `ROUTING_TARGETS` from `routers/inference`, which created a real circular-import bug. Any module wanting to import `capture_invariant` at top-level closed this cycle:

```
services/routing_outcomes -> services/capture_invariant
-> routers/inference (for ROUTING_TARGETS)
-> services/routing_brain
-> services/routing_outcomes ← back here
```

AIN-285 worked around it with two function-local imports (`noqa: PLC0415`). Lifting the constants to a leaf module breaks the cycle for good.

What this PR does

• NEW: `ainfera_api/routing_targets.py` — ~75 lines, zero ainfera_api deps. Owns `INFERENCE_MODEL`, `ROUTING_ALIASES`, `ROUTING_TARGETS`, `CANONICAL_ROUTER`, the three SP-1 aliases, `MITHRIL_MODEL`/`AUTO_MODEL` back-compat names, and a pure `is_routed()` classifier. No FastAPI, no logging, no I/O.
• `routers/inference.py` re-exports the constants + a back-compat `_is_routed` alias so existing callers (anthropic_compat, openai_compat, the test suites) don't churn.
• `services/capture_invariant.py` imports from the leaf module — cycle broken.

What this PR does NOT change

• Public string values (`ainfera-inference` + the three SP-1 aliases) — byte-identical.
• `_is_routed` semantics — leaf vs router agree for every SP-1 target + a vendor-slug sample (explicit test).
• `/v1` OpenAPI surface — route definitions unmoved.
• Disc fix(audit): honest cap + cursor for /v1/audit/public and /v1/audit/{id} #12 invariants — no routing weight / threshold / candidate-set change. Pure structural move.

What this enables (next PR)

When AIN-285 (PR #82) rebases onto this, the two `noqa: PLC0415` lazy imports become plain top-level imports of `get_counter`. One-line cleanup per call site; PR #82's diff shrinks.

Test plan

• `ruff check` + `ruff format --check` + `mypy --strict` clean.
• 550/550 unit + smoke tests green (was 546; +4 from `test_routing_targets_module.py` which locks the leaf-module invariant, the back-compat re-exports, the capture_invariant rewire, and leaf-vs-router `is_routed` agreement).
• CI green on integration suite (no behavior change expected).

🤖 Generated with Claude Code

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Note

Low Risk
Pure refactor with re-exports and tests; no change to routing strings, API routes, or dispatch logic.

Overview
Moves SP-1 routing-target constants (ainfera-inference, alias frozensets, is_routed) out of routers/inference.py into a new leaf module ainfera_api/routing_targets.py, so services can classify routed vs passthrough without importing the FastAPI inference router (and its brain/ORM chain).

capture_invariant.py now imports ROUTING_TARGETS from that leaf module, breaking a circular import (capture_invariant → routers/inference → … → back to services). The inference router re-exports the same names and _is_routed so existing from ainfera_api.routers.inference import … call sites stay unchanged; new code is directed to routing_targets.

Adds tests/unit/test_routing_targets_module.py to guard the leaf has no internal ainfera_api deps, back-compat re-exports, the capture_invariant import path, and leaf is_routed vs router _is_routed parity. Public routing strings and dispatch behavior are unchanged.

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