Add admin-configurable summary system prompt (SummaryProvider currently hardcodes it)

Describe the feature you'd like to request

apps/integration_openai/lib/TaskProcessing/SummaryProvider.php currently hardcodes the system prompt used for text-summarisation tasks:

You are a helpful assistant that summarizes text in the same language as the text.
You should only return the summary without any additional information.

There is no admin UI or config:app:set key to override this prompt. The admin-visible Chat User Instructions for Chat Completions field applies only to the interactive chat-completion path, not to summarisation (which goes through TaskProcessing → SummaryProvider::process() on a separate code path). The max_tokens = 1000 default on this same code path is also very restrictive.

Combined, this produces very short generic summaries. Talk call-recording summaries in particular miss most of the value of the transcript — no structure, no attendees, no action items, no decisions, no next steps.

Every deployment that wants better summaries — richer meeting summaries, per-team wording, per-language variants, non-English house style, industry-specific vocabulary — has to patch PHP directly. That patch is clobbered by every app update and has to be manually re-applied. This is fragile across a fleet and is a real papercut for any admin running integration_openai against a LocalAI-compatible endpoint (LiteLLM, Ollama, self-hosted OpenAI-shape LLMs).

Environment where this was observed:

• Nextcloud 33.0.6 (Hub 26 Winter)
• integration_openai 4.5.1
• Talk 23.0.8
• Backend: LiteLLM proxy fronting local LLM (Qwen 2.5) — url config points at the LiteLLM endpoint

Describe the solution you'd like

Add a Summary system prompt text field to Admin → Connected accounts → OpenAI and LocalAI integration (or wherever fits the app's existing settings UI). The field should:

• Persist to integration_openai app config under a stable key, e.g. summary_system_prompt
• Default to the current hardcoded string, so upgrade is a no-op for existing deployments
• Be read by SummaryProvider::process() in place of the hardcoded string; fall back to the current default when the config key is empty
• Support multi-line input (textarea, not single-line)
• Include short help text explaining the field is used verbatim as the system message

Sketch of the code change in SummaryProvider.php:

// before
$systemPrompt = 'You are a helpful assistant that summarizes text in the same language as the text. '
              . 'You should only return the summary without any additional information.';

// after
$default = 'You are a helpful assistant that summarizes text in the same language as the text. '
         . 'You should only return the summary without any additional information.';
$systemPrompt = $this->appConfig->getAppValueString('summary_system_prompt', $default) ?: $default;

Code Change 2 (related, same file): consider also surfacing max_tokens for summarisation as a UI field. It's already tunable via occ config:app:set integration_openai max_tokens --value=8000, but the pairing (prompt + token budget) is what actually determines summary quality — surfacing both in the same UI section makes them discoverable together.

Broader shape (nice-to-have, out of scope for this ticket): the same "hardcoded prompt with no override" pattern likely applies to other core:text2text:* task types (headline, chat-summary, etc.). A broader "per-task-type prompt templates" epic would be the ideal end state, but even just the Summary field would unblock the most common request today.

Describe alternatives you've considered

1. Patching SummaryProvider.php directly on each server. Works, but the patch is clobbered on every app upgrade. Not maintainable across a fleet, and every admin who wants a better summary has to re-derive the same patch.

2. Using the existing "Chat User Instructions for Chat Completions" field. That field only applies to the interactive chat path, not to TaskProcessing summarisation. Confirmed by inspecting SummaryProvider::process() — the chat instructions config is not consulted there.

3. Using a middleware proxy (LiteLLM, LocalAI itself, etc.) to rewrite the system prompt. LiteLLM can inject or replace system prompts on a per-model basis. This works, but pushes app-level configuration into infrastructure, is invisible to Nextcloud admins, and means every deployment needs a proxy in front of the LLM endpoint just for this one setting.

4. Waiting for a broader "per-task-type prompt templates" feature. Ideal end state, but shipping just the Summary override in the meantime would unblock most of the actual pain today.

Start with apps/integration_openai/lib/TaskProcessing/SummaryProvider.php and trace the existing admin settings and app-config persistence used by integration_openai. Add the summary prompt field with its current default and fallback, then verify SummaryProvider::process() uses it and that the setting supports multiline help text; treat exposing max_tokens as the related optional scope.