feat(licence): verify activation JWT ES256 signature before trusting payload

[revelri]

What

Adds client-side ES256 JWT signature verification to the licence activation path.

• New src/licence/verify.rs: verify_es256 (p256 ECDSA + JOSE r||s signature decoding), embedded P-256 verifying-key constant (ACTIVATION_VERIFYING_KEY_B64, currently PLACEHOLDER), runtime override via STRIVO_LICENCE_PUBKEY. When the key is absent/PLACEHOLDER, verification is skipped with a WARN so dev/local-trial flows still work.
• Wired into routes::licence so every backend token is checked before being written to the cache.
• New deps: p256 0.13 (ecdsa), base64 0.22; Cargo.lock regenerated accordingly.

Addresses the ROADMAP HIGH item: "Verify Licence JWT ES256 signature (TODO(licence-verify)) before any Creator Edition commercial launch."

Verification

• cargo test -p strivo-core --lib licence -> 12 tests pass, 0 failed, including accepts_valid_token, rejects_tampered_payload, rejects_signature_from_different_key, placeholder_key_skips_verification_and_returns_claims (tests generate an ephemeral P-256 keypair and sign a real JWT).
• cargo build -p strivo-web -> clean (licence route wiring compiles).
• Rebased clean onto current origin/main (resolved a sha2 version + new-dep manifest conflict, keeping origin's sha2 = 0.11.0); diff is 7 intended files; Cargo.lock carries the new p256/base64 entries.

Draft: review surface for the auditor; CI gates the merge.

[revelri]

The ES256 verification itself is sound (real p256/ECDSA verification, correct JOSE r||s handling, SPKI DER key parsing). Holding merge on two blockers:

1. CI cargo fmt gate fails on src/licence/verify.rs and a few test helper signatures. cargo fmt --all + commit fixes it — no logic change.
2. Test env-mutation race. The verify tests mutate the STRIVO_LICENCE_PUBKEY process-global env var without mutual exclusion. cargo test runs multi-threaded by default (the "sequential by default" comment is incorrect), so a parallel run reproduces a spurious verification failure in placeholder_key_skips_verification_and_returns_claims. Fix with serial_test::serial on the four verify tests, a Mutex guard, or by factoring a verify_es256_with_key fn that avoids env reads in tests.

Follow-on (not blocking this PR): the verified claim's tier is currently discarded in favour of the request-body tier — worth using the verified value in a later change.

[revelri]

Addresses the two REVISION blockers on this PR (loop pass, independently verified):

• rustfmt gate — cargo fmt --all --check now exits 0 (was failing on verify.rs).
• test env-race (root cause, not a serial band-aid) — factored the crypto core out into verify_es256_with_key(token, key_b64); verify_es256 stays a thin env→const key resolver that delegates. Tests now pass the verifying key directly and no longer mutate the process-global STRIVO_LICENCE_PUBKEY (grep -c set_var/remove_var → 0), so the race is eliminated by construction. verify_es256's runtime contract is unchanged.

Verification: cargo fmt --all --check exit 0; cargo test --lib licence::verify 4/4; 5× --test-threads=16 all green; cargo clippy --lib clean on verify.rs. Diff is src/licence/verify.rs only — no Cargo.toml/lock, no new deps.