Added CODE_CONVENTIONS.md and linters to enforce it, and applied it across the codebase

CODE_CONVENTIONS.md sets out the conventions for Go code in this
repository: naming, layout, comments, errors, logging, configuration and
tests. .golangci.json enables the linters that check the parts that can be
checked (each limited to the rules the conventions use); the rest is
applied by hand.

Applied across the codebase:
- Naming: full-word names in application code, initialisms in one case
  (BuyerID, PrintJSON, PrintYAML, ocdsGetBuyerID), no names shadowing
  built-ins (min/max are minGap/maxGap), mustRaw renamed rawJSON as it
  returns nil rather than panicking, and config.ConfigLevelToLogLevel
  renamed LevelToLogLevel.
- Layout: lines kept to 80 characters where possible, assignments taken
  out of if and for statements that don't fit, wrapped arguments all on
  one line or one per line, steps separated by blank lines, no else after
  a return, function and struct literals wrapped.
- Comments: doc comments on every exported name (including each section's
  Map, Flatten, TypeOf, ValueOf and SetValue), section banners, British
  spelling.
- Errors: a created error starts with its package name, a wrapping error
  adds only what was being done, sentinels in one commented var block,
  errors.Is and errors.As in place of == and type assertions, and no
  err := shadowing of a named result. That fixes the mock server losing
  its access log's close error.
- Logging: the fetcher passes its workers an untagged logger, so
  findatender's lines are tagged with their own component rather than
  with fetcher's as well; cmd/mock-fts logs "stopped" with slog.
- Configuration: util.WithTimeout (moved from the websocket package)
  applies a timeout of 0 or less as no limit, used for the websocket
  write and ping, TCP close and tcpserver shutdown timeouts; every setting
  documents what 0 or empty means, including the rate limit rule's keys
  in internal/config/README.md.
- Tests: related tests merged into table-driven ones with named cases.

Separately from CODE_CONVENTIONS.md, internal/store/README.md gains a soft
SQL style convention for the queries in internal/store/queries.go, and the
queries are reformatted to follow it.

internal/store/queries.go

@@ -18,23 +18,27 @@ package store
 
 // ---------- opportunities ----------
 
-// selectOpportunityLatestReleaseDate reads the date SaveOpportunity compares against.
+// selectOpportunityLatestReleaseDate reads the date SaveOpportunity compares
+// against.
 const selectOpportunityLatestReleaseDate = `
 SELECT latest_release_date FROM opportunities WHERE uuid = ?`
 
-// upsertOpportunity inserts or updates a opportunity. Named parameters (types.Opportunity).
+// upsertOpportunity inserts or updates a opportunity. Named parameters
+// (types.Opportunity).
 //
 // An UPDATE on conflict, not a REPLACE: REPLACE would delete the row first,
 // cascading to the opportunity's releases and changing its id.
 const upsertOpportunity = `
 INSERT INTO opportunities (
 	uuid, uuid_type, title, status, description, buyer_name, buyer_id,
-	main_category, procurement_method, multi_lot, sme_suitable, vcse_suitable,
-	start_date, end_date, gross_value, has_documents, latest_release_date
+	main_category, procurement_method, multi_lot, sme_suitable,
+	vcse_suitable, start_date, end_date, gross_value, has_documents,
+	latest_release_date
 ) VALUES (
 	:uuid, :uuid_type, :title, :status, :description, :buyer_name, :buyer_id,
-	:main_category, :procurement_method, :multi_lot, :sme_suitable, :vcse_suitable,
-	:start_date, :end_date, :gross_value, :has_documents, :latest_release_date
+	:main_category, :procurement_method, :multi_lot, :sme_suitable,
+	:vcse_suitable, :start_date, :end_date, :gross_value, :has_documents,
+	:latest_release_date
 )
 ON CONFLICT (uuid) DO UPDATE SET
 	uuid_type = excluded.uuid_type,
@@ -162,7 +166,8 @@ INSERT INTO opportunity_release_tags (uuid, release_id, tag)
 VALUES (:uuid, :release_id, :tag)
 ON CONFLICT DO NOTHING`
 
-// selectReleaseTagsByOpportunity reads the tags of all of a opportunity's releases.
+// selectReleaseTagsByOpportunity reads the tags of all of a opportunity's
+// releases.
 const selectReleaseTagsByOpportunity = `
 SELECT *
 FROM opportunity_release_tags
@@ -180,13 +185,14 @@ ORDER BY release_id, tag`
 const upsertClassificationCodeReturningID = `
 INSERT INTO classification_codes (is_cpv, code, classification)
 VALUES (:is_cpv, :code, :classification)
-ON CONFLICT (is_cpv, code, classification) DO UPDATE SET is_cpv = excluded.is_cpv
+ON CONFLICT (is_cpv, code, classification)
+	DO UPDATE SET is_cpv = excluded.is_cpv
 RETURNING id`
 
 // ---------- opportunity_classifications ----------
 
-// deleteOpportunityClassifications clears a opportunity's mappings before they are
-// rewritten.
+// deleteOpportunityClassifications clears a opportunity's mappings before they
+// are rewritten.
 const deleteOpportunityClassifications = `
 DELETE FROM opportunity_classifications WHERE opportunity_uuid = ?`
 
@@ -197,8 +203,8 @@ VALUES (?, ?)
 ON CONFLICT DO NOTHING`
 
 // selectClassificationsForOpportunities reads the classifications of several
-// opportunities. "IN (?)" is expanded by sqlx.In, which must be followed by Rebind.
-// c.* must map entirely onto types.ClassificationCode.
+// opportunities. "IN (?)" is expanded by sqlx.In, which must be followed by
+// Rebind. c.* must map entirely onto types.ClassificationCode.
 const selectClassificationsForOpportunities = `
 SELECT tc.opportunity_uuid, c.*
 FROM opportunity_classifications tc