Stop sendNotification being callable from the browser

[doomlab]

Stage 3 (and last of the planned set) of closing the server-side access gap found while tracing #539. Stacked on #560 (which is on #559, on #558); retarget as each merges.

Problem

sendNotification was a Blitz mutation, so any signed-in user could call it directly and create a notification, with an official-looking message, for any user ids they chose, in any project. (Nothing in the browser called it. Only eight server-side mutations did.)

Fix

Rather than adding checks to an endpoint that has no legitimate browser caller, it stops being an endpoint: the code moves, unchanged, to src/notifications/utils/sendNotification.ts as a plain function, and the eight mutations that use it (task, comment, team, invite, project, approval) import it from there. The old file in mutations/ is deleted. Their calls are unchanged (the function still accepts the context argument they pass). The input is still validated against the template schema.

Testing

• sendNotification.test.ts (5): creates the notification with the right recipients/project/route from STAPLE, validates the template data, rejects unknown templates and malformed input without writing.
• A guard test that the endpoint file does not exist (a file in a mutations folder is callable by any signed-in user). I checked it fails if the old file is put back.
• Typecheck (apart from the existing getJsonSchema.test.ts errors), lint and the whole suite (274 tests) pass.
• Not yet checked in the app: the notifications that these actions produce (assigning a task, commenting, adding someone to a team, accepting an invite, creating a project, approving or completing a task log).

Still open from this audit

• The summary viewer build isn't tied to a project/requester (any manager who knew another's random job id could download it).
• Attaching a role or form version to a task isn't checked against ownership.

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