Fetch Details agent (was: Scrape step)

[rafacm]

What to build

Migrate the Scrape pipeline step to a Pydantic AI agent as an SDK swap (single LLM call, no tools), establishing the agent shape so future PRs can grow it. Full rename Scrape → Fetch Details (breaking change) across status enum, file names, env vars, configure wizard, telemetry, docs, and diagrams.

Detailed plan: doc/plans/2026-04-28-fetch-details-agent.md.

Delivered as one PR on feature/fetch-details-agent with two commits:

1. Rename (status enum, files, env vars, configure wizard, recovery can_handle literal, docs, diagrams) + status data migration.
2. Agent migration (introduce episodes/agents/_model.py and episodes/agents/fetch_details.py, refactor fetch_details_step.py to call the agent, delete get_scraping_provider).

Acceptance criteria

• episodes/agents/fetch_details.py exposes EpisodeDetails, get_agent(), run(html) and imports only Pydantic AI / Pydantic / stdlib / agents/_model.py (no Django, no DBOS, no episodes.models).
• episodes/agents/_model.py provides a pure build_model(model_string, api_key) helper; recovery agent refactored to use it.
• scraper.py → fetch_details_step.py; agent.py/browser.py/deps.py/tools.py/resume.py → recovery_*.py.
• Status enum value scraping → fetching_details; data migration updates Episode.status and ProcessingStep.step_name rows.
• Env vars: RAGTIME_SCRAPING_PROVIDER removed; RAGTIME_SCRAPING_MODEL → RAGTIME_FETCH_DETAILS_MODEL (Convention B, e.g. openai:gpt-4o-mini); RAGTIME_SCRAPING_API_KEY → RAGTIME_FETCH_DETAILS_API_KEY. .env.sample and core/management/commands/configure.py updated.
• STEP_FUNCTIONS[Episode.Status.FETCHING_DETAILS] points at episodes.fetch_details_step.fetch_episode_details; PIPELINE_STEPS updated; get_scraping_provider deleted from episodes/providers/factory.py.
• Fast-path skip and empty-field-only merge behavior preserved in the orchestrator.
• Recovery AgentStrategy.can_handle() literal flipped "scraping" → "fetching_details".
• Excalidraw diagrams updated: ragtime-processing-pipeline, ragtime-processing-pipeline-with-recovery, ragtime-recovery (both .excalidraw and .svg).
• Docs: README.md pipeline table, doc/README.md step descriptions, doc/features/2026-04-28-fetch-details-agent.md, planning + implementation session transcripts in doc/sessions/, CHANGELOG.md ## 2026-04-28 entry with **BREAKING** marker.

Out of scope (future work)

• Giving the agent tools (browser use, etc.)
• Migrating the Download step to its own agent
• Deleting the recovery agent
• Unit/eval tests for the agent
• Renaming the Episode.scraped_html model field
• Migrating summarize/extract/resolve/translate/embed steps

Blocked by

None - can start immediately.