fix(content): strip model preamble/meta-chatter from AI summaries (#912)

[laboef1900]

Summary

• AI summaries were persisting the model's raw output verbatim, so framing preambles ("Here's a summary of the article in Markdown format:") and trailing meta/offer lines ("…please provide it so I can respond in German.") got stored and rendered.
• Adds an exported, conservative stripSummaryPreamble() in the summary worker, applied right after the stream is collected and before the empty-check, PII scan, and Markdown→HTML conversion, so both summary_text and summary_html are stored clean.
• Hardens the summarize system prompt to tell the model to output only the summary (defense-in-depth; the strip pass is the guarantee).
• Backend-only: no schema, contract, or frontend change.

Closes #912.

Root cause

summarizePage() stored the collected LLM stream directly, and the summarize prompt — unlike the improve_* prompts — never told the model to omit preamble/closing remarks, so framing and meta-chatter leaked into the stored/rendered summary.

Fix

• New pure helper stripSummaryPreamble(markdown): strips a leading framing line (requires an explicit "here's/below is/the following is … summary …" opener plus a following newline) and any bare "Summary:" heading, then peels trailing lines carrying an explicit offer/meta phrase (e.g. "please provide", "let me know", "respond in German/English").
• Wired in before the empty-check/PII-scan/HTML step; the existing empty-summary guard now also covers a model that returned only preamble.
• Tightened the summarize prompt.

Testing

• TDD: extended backend/src/domains/knowledge/services/summary-worker.test.ts — pure unit tests of the helper (strip leading/trailing, no false-positive on a real "If…" sentence, no-op on clean input) plus a DB-backed wiring test asserting the stored summary_text drops the framing and keeps the real content. Fails before the fix (stripSummaryPreamble is not a function; wiring test stored the framing text), passes after.
• cd backend && npx vitest run src/domains/knowledge/services/summary-worker.test.ts (24 pass) - eslint (pass) - tsc --noEmit (pass)

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