test: cover Discord alert delivery, which had never been executed

[FMSMITH91]

send_discord had 0% coverage. It's the path a "went offline unexpectedly" alert takes, and a broken one fails exactly like no alert at all — silence. That's the same failure mode as the monitor bug this session opened with, and the reason it went unnoticed for weeks.

14 checks, HTTP layer replaced so nothing leaves the machine:

delivery	a valid webhook sends — to discord.com and nowhere else, as JSON, with the message as the payload's content
truncation	5000 chars cut to 1900. Discord rejects over 2000, and a rejected alert is a missed alert
refusal	seven non-webhook strings (evil host, plain http, a discord.com look-alike, non-numeric id, empty, None) are refused before any request is attempted
diagnosis	"couldn't reach discord.com" and "Discord rejected it" stay distinct — different problems for whoever's working out why an alert never arrived
SSRF	_post refuses a URL outside the provider allow-list, asserted directly against a link-local metadata address

Mutation-verified:

drop the truncation to 1900        -> 1 fail
accept any URL as a webhook        -> 5 fail
remove the _post allow-list (SSRF) -> 2 fail
collapse the two failure reasons   -> 1 fail

unit 839 → 854 · smoke 307 · lint clean.

🤖 Generated with Claude Code

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Coverage ∅ diff coverage · +0.04% coverage variation

Metric	Results
Coverage variation	✅ +0.04% coverage variation (-1.00%)
Diff coverage	✅ ∅ diff coverage

View coverage diff in Codacy

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (7e2ace5)	11965	5817	48.62%
Head commit (16a58eb)	11965 (+0)	5822 (+5)	48.66% (+0.04%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#71)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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