feat: migrate to Fess 15.8

[marevol]

Moves this deployment to Fess 15.8.0 / OpenSearch 3.8.0, and brings along the pieces that do not follow those pins on their own.

Changes

Version pins — .env: FESS_VERSION=15.8.0, OPENSEARCH_VERSION=3.8.0 (Fess 15.8.0 targets OpenSearch 3.8.0, per fess-parent-15.8.0). render-fess-config.sh re-fetches the fess_config.properties base from the fess-15.8.0 tag with no change needed: every key the codesearch overlay redefines still exists in the 15.8 base. The api.v2.* → api.* renames in 15.8 do not touch any key the overlay owns.

Plugin download — bin/setup.sh pins fess-ds-git 15.8.0 and downloads from https://maven.codelibs.org/release. Fess plugins are no longer published to Maven Central from the 15.8 line on, so the old URL 404s for 15.8.0 and setup.sh would abort. FESS_PLUGIN_REPO_URL overrides the repository for pinning an older plugin release (≤ 15.7.0, still on Central).

Index schema — fess_indices/_codesearch is a hand-maintained fork, so it does not track the version. Refreshed with what upstream added in 15.8:

• fess.json: index.knn, index.knn.derived_source.enabled, index.merge.policy
• fess/doc.json: content_chunk_vector (nested knn_vector), content_chunk_status

The fork is now identical to codelibs/fess@fess-15.8.0 apart from the codesearch tuning it exists for (line_number_filter, operator_filter / dotnum_filter / code_stop_filter, the seven codesearch document fields, important_content unindexed, no content_minhash_bits copy).

Codesearch does not use semantic search — the Content Chunk Vector Indexer job and content_chunker.search.enabled are both off by default in 15.8 — but a mapping is only ever applied to a brand-new index, so carrying it now is what keeps enabling it later from requiring a full re-crawl. It does make the k-NN plugin a hard requirement; ghcr.io/codelibs/fess-opensearch bundles it.

bin/verify.sh fix — the search API probe read record_count at the top level of the response, but /api/v2/search has always wrapped its payload in a {"response": {...}} envelope. The probe therefore never found a count and silently SKIPped — the one check that catches the "HTTP 200 with 0 hits" failure mode this script exists for (#3 / Redmine 6708). Now reads through the envelope, with a fallback to the flat shape.

Docs — README version references, the plugin-repository change, the 15.8 index-schema note, and an explicit warning that an in-place upgrade keeps the old index mapping (Fess applies a mapping only to an index that has none yet).

Verification

Run against an isolated stack (ghcr.io/codelibs/fess:15.8.0 + ghcr.io/codelibs/fess-opensearch:3.8.0, separate compose project / container names / ports) built from exactly this branch's tracked files.

• bin/setup.sh completes: fess-ds-git 15.8.0 downloaded from maven.codelibs.org, theme packaged from fess-themes main (codesearch 1.0.9), base config fetched from codelibs/fess@fess-15.8.0.
• First boot creates the index with the codesearch schema. Live settings: index.knn=true, index.knn.derived_source.enabled=false, index.merge.policy.floor_segment=16mb, max_merge_at_once=30. Live mapping: all 7 codesearch fields plus content_chunk_status and content_chunk_vector (dimension=768, engine=lucene, space_type=cosinesimil, method=hnsw). No reindex needed.
• bin/verify.sh: all checks pass, including the codesearch schema has every core field of codelibs/fess@fess-15.8.0 and the now-live API probe (the theme's facet request returns hits (18 for 'fess')). The one remaining WARN (duplicate dynamic_templates names lang_ca / lang_en / lang_hu) is pre-existing and unrelated.
• Registered two repositories with bin/register_github.sh and crawled: 20 documents, domain/organization/repository/filetype all populated.
• Search: /api/v2/search returns real facet payloads for organization, repository, filename, filetype; the theme's facet qualifiers work (repository: → 10, filetype:java → 5, organization: → 18). An unsupported facet field correctly answers HTTP 400 invalid_request.
• SPA renders on 15.8 (headless): 18 result cards matching record_count, org/repo/path crumbs, filetype badges, the facet rail with counts, and line-number gutters — confirming the codesearch line_number_filter and appendLineNumber handler still line up. Home and /help render clean; /search/advance serves the SPA (/advance is not an allowlisted theme UI prefix, and the theme links to the canonical path).
• Content Chunk Vector Indexer present and available=false, as expected.
• Zero ERROR lines in fess.log across boot, crawl and search.