feat(pipeline): VEP database mode, and a pipeline-specific database URL

Makes a real annotation runnable locally without the 25 GB VEP cache, which is what
the demo needs and what a reviewer can reproduce in minutes.

- params.vep_database (VEP_DATABASE=true) queries Ensembl's public database instead of
  a local cache. Slower per variant and fewer fields, so --everything is swapped for the
  flags the loader actually stores. Its cache placeholder is NO_CACHE, not NO_FILE:
  Nextflow rejects two staged inputs sharing a filename.
- PIPELINE_DATABASE_URL is handed to the pipeline when set. The loader runs inside a
  container, where the API's own localhost URL would point at the container itself.
- README: how to run the UI's annotate button locally against host Nextflow + Docker.

Verified end to end on pipeline/tests/data/tiny.vcf: bcftools norm split the multiallelic
record, VEP 113 annotated 4 variants live, the loader wrote them and marked the job
succeeded, and the UI shows them. The deletion came back as 22:42126611 CT>C with exact
VCF alleles, which is the case the audit's ID-tagging fix exists for.

Tests: api 51, loader 16, stub run 3/3; ruff, mypy clean.

pipeline/modules/vep.nf

@@ -17,10 +17,14 @@ process VEP {
         "--plugin CADD,snv=${plugin_data}/${params.cadd_snv},indels=${plugin_data}/${params.cadd_indels}",
         "--plugin AlphaMissense,file=${plugin_data}/${params.alphamissense}",
     ].join(' ')
+    // --everything needs the cache (it implies --af_gnomade and friends); the database offers a
+    // smaller set, but still the consequence, gene, HGVS and ClinVar fields the loader stores.
+    def source = params.vep_database ? "--database" : "--cache --offline --dir_cache ${cache}"
+    def fields = params.vep_database ? "--symbol --hgvs --canonical --biotype --variant_class --check_existing" : "--everything"
     """
     vep -i $vcf -o ${vcf.simpleName}.vep.tsv --tab \\
-        --assembly ${params.assembly} --cache --offline --dir_cache ${cache} \\
-        --everything --pick ${plugins} \\
+        --assembly ${params.assembly} ${source} \\
+        ${fields} --pick ${plugins} \\
         --stats_file ${vcf.simpleName}.vep_summary.html --fork ${task.cpus}
     """