feat: construct stitch payload with a non-ad bumper segment when bumperUrl is present

[birme]

Once the webhook interface carries bumperUrl/bumperDurationMs, build the stitch payload so a bumper is inserted as a non-ad segment (no ad-avail markers), distinct from an ad break.

BLOCKED BY: Eyevinn/lambda-stitch#18 ("Stitch segment that is not a break"), which is still OPEN and not yet shipped. lambda-stitch main currently only supports a breaks[] array; the proposed segments[] array does not exist yet. Do not start until lambda-stitch#18 is merged and the deployed stitch endpoint accepts segments.

Scope once unblocked:

• Extend StitchPayload in src/plugins/interface.ts with an optional segments?: StitchPayloadBreak[] (matching the shape lambda-stitch#18 lands on).
• In src/plugins/utils.ts, extend/refactor getVodUrlWithPreroll (or add a sibling helper) so that when a bumper is present it is emitted in segments rather than breaks, while preroll continues to go into breaks (or confirm preroll's correct placement).
• Define and document bumper placement semantics: pos of the bumper (0 vs. after preroll), and how bumper + preroll + ad breaks combine, per the example payload in lambda-stitch#18.
• Wire getNextVod in src/plugins/plugin_webhook.ts to pass bumper fields into the payload builder.
• Add tests in src/plugins/plugin_webhook.test.ts asserting the serialized payload contains the bumper as a segment (not a break) and no ad-avail treatment.

Context: required to resolve EyevinnOSC/community#30.

Split from #47 by daily-backlog-pr triage.

[birme]

Still blocked. Verified on this backlog pass that the external dependency Eyevinn/lambda-stitch#18 ("Stitch segment that is not a break") is still OPEN and unshipped — the deployed stitch endpoint does not yet accept a segments[] array (only breaks[]). Per this issue's own instruction, work must not start until lambda-stitch#18 is merged and deployed.

No code change made. Leaving in Ready (unassigned) so it is re-evaluated on the next pass once lambda-stitch#18 lands.