fix(config): bind all defaulted keys to env vars; doc GOFLAGS=-tags=sqlite_fts5 for go install

[aksOps]

Summary

Two bugs reported by a user installing v0.1.3 in a corporate environment.

Bug 1 — DOCSIQ_LLM_* env vars silently ignored without a config file.
Viper's AutomaticEnv() + Unmarshal() only reads env for keys that are explicitly bound via BindEnv (or already present in the merged config tree) — see Viper #761. Of all the LLM keys, only llm.call_timeout was bound, so DOCSIQ_LLM_PROVIDER, DOCSIQ_LLM_OLLAMA_*, DOCSIQ_LLM_AZURE_*, DOCSIQ_LLM_OPENAI_* were dropped for users without ~/.docsiq/config.yaml.

Fix: walk v.AllKeys() after all SetDefault calls and BindEnv each. The existing explicit BindEnv list stays — it documents the DOCSIQ_API_KEY alias for server.api_key and double-binds harmlessly.

Bug 2 — go install ...@v0.1.3 produces a binary that fails at runtime.
The user hits unable to open store: no such module: fts5 because mattn/go-sqlite3 only enables FTS5 with -tags=sqlite_fts5. The Makefile sets that tag; go install doesn't pick up Makefile tags. Docs-only fix: docs/getting-started.md now shows GOFLAGS='-tags=sqlite_fts5' go install ...@latest with a one-paragraph explanation of why the tag is required.

Files changed (3)

• internal/config/config.go — for _, key := range v.AllKeys() { _ = v.BindEnv(key) } after defaults.
• internal/config/config_test.go — new TestLoad_EnvOverridesLLM regression covering provider, Ollama, Azure, DataDir without a config file.
• docs/getting-started.md — go install line uses GOFLAGS='-tags=sqlite_fts5'; build-from-checkout go build line gains the tag for consistency.

Test plan

• make vet clean.
• make build clean.
• make test green; new TestLoad_EnvOverridesLLM passes.
• End-to-end smoke against a fresh binary on :37789 with DOCSIQ_LLM_PROVIDER=ollama DOCSIQ_LLM_OLLAMA_EMBED_MODEL=bge-m3 and no config file. Server log shows ⚙️ LLM provider initialised provider=ollama model=bge-m3 — confirms the env value flows into the resolved config.
• Production server on :37777 was not touched.

Out of scope

• Vendoring (go mod vendor) — v0.2.0.
• Swapping mattn/go-sqlite3 for modernc.org/sqlite — separate, larger change.
• The documents.file_hash UNIQUE collision on --force, case-sensitive /api/graph/neighborhood, community-finalize duplicate rows, and Upload SSE 202-vs-partial-fail status — all tracked separately.

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