fix(settings): merge partial /api/settings updates instead of overwriting

[richiejp]

Description

Merge partial settings into the runttime settings map instead of overwriting it.
This prevents some endpoints from clobbering the current settings.

A side effect is that now to unset something it has to be present in the JSON and
set to null. This is how all of the present callers do it already, but it could effect
external API consumers that rely on the old behaviour.

Notes for Reviewers

POST /api/settings rebuilt runtime_settings.json from only the request
body, so a focused admin page that submits a single field wiped every
other persisted setting. The Middleware proxy tab (mitm_listen) and
detector table (pii_default_detectors), plus the MCP SetBranding tool
(instance_name/instance_tagline), all POST partial bodies; the
no-omitempty api_keys and pii_default_detectors fields even round-tripped
as JSON null.

Read the persisted settings and overlay only the fields the request set
(RuntimeSettings.MergeNonNil) before writing. Every field is a pointer, so
the reflection-based merge is total over the struct and any field added
later is preserved automatically. Absent or null fields are now kept;
clearing a setting is done by sending its explicit empty/zero value
(api_keys [], mitm_listen "", etc.), unchanged from before. The full
Settings page sends every field, so its Save behaves identically.

Assisted-by: Claude:claude-opus-4-8 Claude-Code
Signed-off-by: Richard Palethorpe io@richiejp.com

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