docs: update SQL-to-FalkorDB migration guide — BigQuery/Spark, scaffolding, security fixes

[Copilot]

PR #402 refreshes the SQL-to-FalkorDB migration guide but removes UI screenshots and ships two issues flagged in review. This PR applies all content updates, restores the screenshots, and addresses both reviewer concerns.

Content updates

• New sources: BigQuery and Spark SQL (via Livy) added throughout (description, prerequisites, CLI sections, metrics ports, purge/scaffold support matrices)
• Schema introspection + scaffolding: new section covering --introspect-schema and --generate-template; supported by all 9 sources
• Control plane: expanded feature list (schema preview, topology visualization, state/watermark inspection, persisted logs); full 14-endpoint API reference replaces the prior 2-endpoint stub
• Per-loader capability notes: Databricks/Spark (one-shot only), PostgreSQL (daemon, no purge), BigQuery (purge supported)

Bug fixes from review

• Duplicate metrics ports: BigQuery and MySQL both default to 9995; MariaDB and Spark both default to 9997 — explicit collision warning added with guidance to set distinct ports when running either pair concurrently
• SSE auth security: query-string token fallback note expanded to call out leakage vectors (browser history, access logs, Referer header) and list concrete mitigations (HTTPS, short-lived tokens, log filtering, cookie/session/polyfill)

Marketing/product

• Screenshots restored: PR docs: update SQL-to-FalkorDB migration guide for latest feature set #402 dropped 4 control-plane UI screenshots (tools menu, run execution, log view, metrics view) — kept here as they demonstrate product UX

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