feat(canary): API-trashing defense — 4-layer fingerprint validation

Adds per-mint nonce gating, structural shape validation, mint UUID
consistency checks, and a per-(token, IP) rate limiter to the canary
worker so attackers who extract a canary from a decky filesystem cannot
poison fingerprint forensics by replaying or forging ?d= submissions.

Changes:

base.py
  fingerprint_nonce: Optional[str] added to CanaryArtifact so generators
  can surface the nonce to the cultivator without coupling the generator
  directly to DB code.

obfuscator.py
  nonce_for(callback_token, mint_uuid): HMAC-SHA256 keyed on
  DECNET_CANARY_FINGERPRINT_SECRET, truncated to 16 hex chars.
  FingerprintSecretMissing raised at mint time if env var is unset.
  render_fingerprint_js() now accepts nonce= and substitutes MINT_NONCE.

fingerprint_payload.js
  New MINT_NONCE placeholder. Appended as &k= on all beacon URLs (bare-open,
  single-shot, chunked). Using &k= avoids colliding with &n= (chunk total).

fingerprint_html.py / fingerprint_svg.py
  Derive nonce via nonce_for() and pass to render_fingerprint_js(). Set
  artifact.fingerprint_nonce so the cultivator can persist it.

cultivator.py
  Passes fingerprint_nonce into create_canary_token() when present on the
  artifact; NULL for all non-fingerprint generators.

canary.py (model)
  fingerprint_nonce: Optional[str] = Field(default=None, max_length=16)
  added to CanaryToken. None for non-fingerprint tokens.

worker.py
  _extract_fingerprint now returns (meta_dict, parsed_fp) tuple.
  _record_hit accepts parsed_fp + raw_nonce and runs 4 layers after
  token lookup: nonce match, shape check, mint UUID consistency, rate limit.
  Each failure sets _fp_invalid_* flag and drops structured _fp.
  Trigger row always lands regardless.

tests/canary/conftest.py
  Session-scoped autouse fixture sets DECNET_CANARY_FINGERPRINT_SECRET so
  fingerprint generator and worker tests work offline.

tests
  5 new worker HTTP tests and 2 new generator tests covering each
  validation layer.

tests/canary/conftest.py

@@ -9,11 +9,29 @@ both accept a stripped-down skeleton with just ``[Content_Types].xml``,
 from __future__ import annotations
 
 import io
+import os
 import zipfile
 
 import pytest
 
 
+@pytest.fixture(autouse=True, scope="session")
+def _canary_fingerprint_secret():
+    """Ensure DECNET_CANARY_FINGERPRINT_SECRET is set for all canary tests.
+
+    Fingerprint generators call nonce_for() which raises if the env var
+    is unset. A test-only sentinel value is fine — it just needs to exist.
+    """
+    key = "DECNET_CANARY_FINGERPRINT_SECRET"
+    prev = os.environ.get(key)
+    os.environ.setdefault(key, "test-secret-for-canary-tests-only")
+    yield
+    if prev is None:
+        os.environ.pop(key, None)
+    else:
+        os.environ[key] = prev
+
+
 _DOCX_CONTENT_TYPES = (
     '<?xml version="1.0" encoding="UTF-8" standalone="yes"?>'
     '<Types xmlns="http://schemas.openxmlformats.org/package/2006/content-types">'