fix(web): bound the free-text context fed into the research prompt

[SEPURI-SAI-KRISHNA]

Summary

/api/onboarding/research interpolates the caller-supplied name and email straight
into the prompt sent to grok-4-fast:

const contextParts: string[] = []
if (name) contextParts.push(`Name: ${name}`)
if (email) contextParts.push(`Email: ${email}`)
const userContext =
    contextParts.length > 0
        ? `\n\nAdditional context about the user:\n${contextParts.join("\n")}`
        : ""

ResearchRequest declares both as string, but nothing enforces that at runtime — the
values come from await req.json(). Two consequences:

• Unbounded length. Neither field is capped, so an arbitrarily large string is billed
  as prompt tokens on a model call that also runs the web_search and x_search tools.
  xUrl is already constrained (the derived handle must match
  /^[A-Za-z0-9_]{1,15}$/); these two fields are the remaining unbounded input.
• No type guard. A non-string value is template-stringified, so an object arrives in
  the prompt as Name: [object Object].

Changes

Route both fields through a sanitizeContextField helper that:

• returns "" for anything that is not a string, so only strings reach the prompt;
• collapses runs of whitespace to a single space;
• trims and truncates to MAX_CONTEXT_FIELD_LENGTH (200).

The whitespace collapse matters beyond tidiness: each value is meant to occupy one
Name: / Email: line, and an embedded newline would let it forge an additional line of
prompt instead of staying inside its own field.

To be explicit about what this does not do: it bounds cost and keeps each value on
its intended line, but it is not a defence against prompt injection. 200 characters is
ample for an injected instruction, and the fields are free text by design.

Note for reviewers: this route appears to be unused

I could not find any caller for this route in the monorepo — the only /api/* route the
web client fetches is /api/og (apps/web/components/memories-grid.tsx:157). The same
looks true of /api/onboarding/extract-content and /api/onboarding/account-status. All
three were added in #672 (Jan 2026), and onboarding has been rebuilt since — #904
(remove unused old onboarding flow), #1067, #1178 — which appears to have dropped the
callers while leaving the routes.

I have only grepped this repository and cannot rule out a caller outside the monorepo,
which is why this hardens rather than removes. If these routes are dead, deleting all
three would be the better fix, and I am happy to open that PR instead. Same note appears
on #1528, which bounds the sibling extract-content route.

Testing

biome check passes on the changed file. apps/web has no test runner configured (no
vitest dependency, no test script), so no tests were added.