MySQL-compatible tag upsert, direct driver deps, multi-DB docs and compose verification

[joestump]

Part of #302 (EPIC: Multi-Database MariaDB Compatibility). Extends #307. Governing: ADR-0023, SPEC-0014 (multi-database-support). Depends on: #307 (MariaDB DDL), #324 (Lidarr-optional unblocks the compose MUST-start scenarios).

The audit found MariaDB is broken at three layers beyond #307's DDL scope — a SPOTTER_DATABASE_DRIVER=mysql deployment currently exits at startup, and would fail on first tag write even after DDL lands:

Requirements

• Tag upsert SQL is Postgres/SQLite-only (audit CRITICAL): $1..$6 placeholders + ON CONFLICT ... DO NOTHING — MySQL needs ? placeholders and INSERT IGNORE/ON DUPLICATE KEY (internal/tags/upsert.go:87-97). Make it dialect-aware
• go.mod: lib/pq and go-sql-driver/mysql are in the indirect block despite blank imports; move to direct requires per SPEC-0014 (go mod tidy) — audit WARNING
• Docs (audit CRITICAL): docs-site configuration reference documents only sqlite3/postgres — add mysql/mariadb driver + DSN examples, the pure-Go/no-CGO note, and link docker-compose.mariadb.yml from docker.md (docs-site/docs/getting-started/configuration.md:48-55,189-195; docker.md:15-26)
• Verify the SPEC-0014 compose scenarios end-to-end: docker compose -f docker-compose.mariadb.yml up starts, connects, and completes a tag write (CI job or documented manual verification)

Acceptance Criteria

• Fresh MariaDB deployment boots, migrates, and persists tags
• Docs cover all three drivers with working DSN examples

Branch

feature/346-mysql-upsert-docs-compose

PR Convention

PR title references the story; body ends with Closes #346, references the parent epic and governing artifacts above.

🤖 Posted on behalf of @joestump by Claude.