Correct developer and feature pages against the Postal 3.3.7 source

[benrfairless]

Reviews the Developer and Features sections against the Postal source at tag 3.3.7 and corrects the places where the documentation had drifted from the code, filling in behaviour that was undocumented along the way.

Corrections

Page	Key changes
Webhooks	Every webhook body is an envelope {event, timestamp, uuid, payload}; the docs showed the inner payload as the top level. Adds the signature headers (X-Postal-Signature-256, legacy X-Postal-Signature, X-Postal-Signature-KID) and the /.well-known/jwks.json endpoint for verifying them, the 5 s timeout, the retry schedule (6 attempts: 2/3/6/10/15 min), the SendLimitApproaching/SendLimitExceeded events, and corrects history retention from 7 to 10 days.
Receiving e-mail by HTTP	The status-code handling was inverted: 5xx, timeouts and connection errors are soft failures that are retried, other 3xx/4xx are hard failures, and 429 hard-fails without a bounce. Timeout is 5–60 s per endpoint rather than a fixed 5 s. Adds missing payload keys (reply_to, replies_from_plain_body, rcpt_to/mail_from on the raw payload), form-data attachment flattening, the back-off formula, and the 3.3.7 SSRF block list with allowed_request_destinations.
Using the API	Expanded into a reference for all four /api/v1 endpoints. Documents the third parameter-error status, every error code, all parameters (incl. the headers key, comma-separated recipients, the 50-per-list limit, rcpt_to must be an array for send/raw), and the full _expansions response shape.
OpenID Connect	User matching order was reversed: Postal matches on OIDC UID + issuer first, then falls back to e-mail address (only for users not yet linked). Notes that linking clears the local password, adds the manual-endpoint config for discovery: false.
Spam & Virus Checking	Adds rspamd (which takes precedence over spamd when both are enabled) and ClamAV configuration. Corrects threshold semantics (> spam threshold, >= failure threshold), documents the X-Postal-Spam*/X-Postal-Threat headers, and that a virus detection only sets a flag – it never holds or fails a message. Outbound scanning only happens when an outbound threshold is set.
SMTP Authentication	Username handling per mechanism (ignored for PLAIN/LOGIN, org/server for CRAM-MD5), From-then-Sender validation order, SMTP-IP longest-prefix matching, credential hold, a table of SMTP responses, CRLF-only end-of-DATA.
SMTP TLS	STARTTLS only (no implicit TLS/465), certificate file may contain the chain, container paths.
Click & Open Tracking	Rewriting only happens once the tracking domain's CNAME check is OK; paths served; 307 redirect; excluded domains are exact-match.
Health & Metrics	Full list of exported Prometheus metrics with labels; / endpoint.
IP Pools	IP address fields (hostname is used for HELO; priority weighting), rule matching (exact address/domain, server rules before organization rules), meaning of the default pool, and that messages allocated to an IP no running worker holds are never processed.
Logging	All logging.* options, SMTP connection logging, per-server Log SMTP data.

Also fixes a typo ("owned by that domain" → server) and adds description frontmatter to pages that had it empty.

Stacked on #90 and #91 (cross-links to the new pages). pnpm generate passes on this branch (177 routes prerendered, no 404s from the link crawler).